Marine Stocking in Victoria

A Preliminary Assessment of the Potential Suitability of Victorian Waters Selected for Fish Releases

Dr. Matthew D. Taylor
School of Biological Earth and Environmental Sciences
University of New South Wales

This report is prepared on behalf of UNSW Global Pty Limited

02 February 2010

Authorised Contact: Vinita Chanan

T: +61 2 9385 3175
F: +61 2 9662 6566
E: v.chanan@unsw.edu.au
W: www.consulting.unsw.edu.au

PO Box 6666
UNSW NSW 1466
AUSTRALIA

Table of Contents

Scope of the assessment

A brief review of the ecology of species proposed for release in Victorian estuarine waters
Mulloway
Black bream
Estuary perch
Dusky flathead
Eastern king prawn

A review of potential opportunities and the need for releases of selected fish species into selected Victorian estuarine waters
Snowy/Brodribb RiverLake Tyers
Anderson Inlet
Patterson River
Werribee River
Anglesea River
Painkalac Creek
Barham River
Curdies Inlet and Curdies River
Hopkins River
Merri River
Lake Yambuk

Summary and recommendations for further research
Further development of a marine stocking program in Victoria
Conclusions
Acknowledgements
References

Scope of the assessment

The objective of this study is to conduct a preliminary assessment of the capacity of selected Victorian marine or estuarine waters to support stockings of selected species identified by stakeholders as desirable for recreational fishing purposes. In line with the recommendations of the responsible approach to marine stock enhancement (Blankenship and Leber, 1995), Fisheries Victoria undertook two workshops with key stakeholders in Victorian fisheries to establish a context for further investigation of marine stocking in Victorian waters. During these workshops stakeholders were given the opportunity to recommend, justify and challenge the selection of preferred recreational target species and key waters along the Victorian coast, as well as discuss the benefits and risks associated with releasing the proposed species in the proposed waters. This qualitative process resulted in a list of stakeholder preferred species and waters (all estuaries) for further investigation of enhancement potential.

The current preliminary assessment of marine stocking potential included visual, on-water inspection of the proposed estuaries to determine morphological, physical, environmental, habitat and ecological characteristics. This information was then synthesised with a detailed review of the known biology, ecology and fisheries of the selected potential stocking species, to provide an appraisal of the suitability of the species – estuary combinations determined in the initial workshop phase. This report represents the collation and discussion of data from the preliminary assessment, and provides recommendations on species-estuary combinations that are potentially suitable for fish releases and warrant more detailed investigation. This short list of species – estuary combinations will be considered for the next step of the program, where a more comprehensive assessment of costs, benefits and risks associated with particular stocking proposals will be undertaken in accordance with Victorian and national policies for the translocation of live aquatic organisms, and using an appropriate risk assessment framework. This process will include investigations of the risk of adverse disease, genetic and ecological impacts of fish releases, and how to minimise these risks.

This report also deals with additional aspects relevant to the objectives, including examination of existing Catchment Management Authority assessments, identification of additional research requirements, and estimates of the costs associated with further steps in the process of developing an enhancement program. Actual habitat mapping, assessment of the carrying capacity of particular waters, and assessment of projected returns from stocking of particular target species in particular waters are beyond the scope of this assessment.

The preferred species – estuary combinations identified at stakeholder workshops and listed in Kramer (2009) were refined by the Department of Environment, Land, Water and Planning DELWP (former) Department of Environment and Primary Industries Victoria using additional aquatic resource management criteria not addressed during the workshop phase, and a revised list was provided for consideration in this preliminary assessment (Table 1)

Table 1. Refined list of stakeholder preferred species-estuary stocking combinations identified for assessment as part of this scoping study.

¹ Western Victorian estuaries are not within the natural geographic range of any eastern Australian prawn species (Kailola et al., 1992). The western king prawn M. latisulcatus has occasionally been recorded from western Victorian coastal waters (Winstanley, 1975), but there are no confirmed records of M. latisulcatus occurring in Lake Yambuk or the Anglesea River.

² Lake Tyers is within the natural geographic range of eastern king prawn Melicertus plebejus (Kailola et al., 1992), and the species has been captured there previously in good numbers. Lake Tyers is also within the natural geographic range of the school prawn Metapenaeus macleayi and greasyback prawn Metapenaeus benettae (Kailola et al., 1992).

Recruitment limitation and justification of fish releases to enhance fisheries

Many factors have been proposed to justify stock enhancement and restocking, however fish releases are only feasible under two ecological scenarios: 1) available habitat is not sufficiently colonised with recruits through natural spawning events; and 2) trophic resources are available with that habitat to support additional recruits, without displacing competitors or wild conspecifics (Munro and Bell, 1997; Taylor et al., 2005). With the exception of exceptional recruitment events, abundances of most species are thought to be limited by adequate supply and successful settlement of juveniles into available key habitat (Doherty and Williams, 1988), as opposed to food limitation and starvation. The production of exceptional year classes provides some evidence to support a general hypothesis that marine systems rarely reach carrying capacity. This is further supported by the fact that fisheries harvest commonly leads to a decline in population abundance, despite the fact that mass balance and trophic models indicate that <10 % of aquatic primary productivity is required to sustain current harvests (meaning that marine populations are unlikely to be food limited, particularly at lower trophic levels, Pauly and Christensen, 1995). This information points to an idea that fisheries are limited by either insufficient supply of new recruits, or limited juvenile habitat to support new recruits through vulnerable life history stages. This is a general suggestion and there are of course exceptions (e.g. Svåsand et al., 2000), but this preliminary assessment of the suitability of selected Victorian estuaries to receive stockings of selected fish species will focus on the potential limitations in the supply of wild recruits, and the availability of habitat to support hatchery reared recruits.

Where the supply of new recruits (i.e. larvae or juveniles) is not sufficient to fully or optimally utilize the resources available in a particular system, the population is referred to as being recruitment limited. This phenomenon is referred to as recruitment limitation, and is most intensively studies in coral reef systems. An association between freshwater flows and estuarine fisheries exists, albeit poorly understood, but it is likely that estuarine species endemic to eastern and south-eastern Australia have evolved in response to high variability and strong seasonal rainfall. As a consequence, certain life history stages may be dependent on the timing and magnitude of freshwater inflow into estuaries. Evidence for this arises from descriptive and semi-quantitative studies presenting positive (and sometimes negative) associations between freshwater flow and estuarine fisheries, however these studies often fail to empirically evaluate the underlying causes (Gillanders and Kingsford, 2002). In these studies, fishery impacts usually manifest either immediately following a freshwater pulse (which may reflect changes in catchability), or over a 'lag-period' which usually equates to the period of time between larval recruitment and appearance in the catch (Meynecke et al., 2006). A 'lagged' association reflects the conditions during the first year of life which stimulate successful recruitment and settlement (Robins et al., 2005). For example, in mulloway Argyrosomus japonicus a freshwater related signal may result in a greater number of new recruits to estuarine nursery areas (Ferguson et al., 2008). Also, the catch per unit effort by gillnets of dusky flathead Platycephalus fuscus and mulloway decreased during drought periods over 10 years, across 7 NSW estuaries (Gillson et al. 2009). These examples provide evidence to support a link between the timing of freshwater flows, where subsequent ecological processes link these with fisheries catch through a match-mismatch mechanism (Cushing, 1990) between larval/juvenile supply, estuarine production and coastal oceanographic processes. Evaluation of this match-mismatch scenario in the context of stock enhancement has not yet been tested, and hypotheses regarding the nature and occurrence of recruitment limitation in estuaries are often extended from research in coral reefs and marine systems. Empirical evaluation of recruitment limitation within Australian estuaries is clearly warranted, and would benefit both our understanding of estuarine ecology, and how fish releases may be employed to enhance estuarine fish stocks.

In summary, in the context of the review of species biology below, stock enhancement may be justified from an ecological perspective within Victoria's estuaries where:

1. Upstream freshwater extraction or barriers to flow removes or decreases the seasonal penetration of substantial freshwater flows into estuaries, where evidence exists that a species may rely on such an event to fulfil certain life history stages. Some examples include;

• A diadromous species requiring certain hydrological conditions (i.e. a substantial salt wedge; or brackish water) to hatch or complete larval development
• A species that relies on a strong freshwater signal to stimulate a spawning event
• A coastal spawning species with pelagic larvae that is unable to settle into key habitat within estuaries due to the absence of a recruitment signal

2. Physical barriers that prevent access and settlement to key early juvenile habitat, or adult spawning habitat. Some examples include;

• A diadromous species unable to access freshwater habitat due to the presence of a weir with no provision for fish passage
• A coastal spawning species which is unable to reach coastal waters to spawn due to the presence of a sandy berm
• A coastal spawning species with pelagic larvae that is unable to settle into key habitat within estuaries due to the presence of a sandy berm

3. Important key habitat for larval or early juvenile survival is no longer present in the estuary, in which case organisms may be released at a size greater than the threshold where such habitat ceases to be essential. Some examples include;

• A species that relies on certain structured habitat to lay eggs, or provide refuge for larvae, where that habitat no longer exists
• A species that requires certain hydrological conditions during early life history stages, where those conditions do not exist either through limited freshwater inflow, or limited tidal penetration.

A brief review of the ecology of species proposed for release in Victorian estuarine waters

A brief review of the ecology of the proposed species is necessary to allow consideration of the suitability of certain water bodies to receive additional hatchery-reared recruits. Key aspects of the species biology and ecology contained in this brief review include dietary requirements, limitations and tolerance of hydrological conditions, key habitat requirements, and distributional information. An estimate of the natural mortality to growth ratio is included where estimates of mortality and growth are available or can be calculated. A low mortality to growth ratio makes the species potentially more suitable for enhancement, as these species represent lower losses to predation and faster progression to the fishery (Munro and Bell, 1997; Taylor et al., 2005). A brief appraisal of knowledge gaps for each species is also included in relevant sections throughout the report. Lack of knowledge on genetic stock structure is an issue for many marine species in Australia, and will need to be addressed during the development of any release program where the information is lacking. Research on stock structure of mulloway Argyrosomus japonicus and eastern king prawn Melicertus plebejus is currently underway (P. Mather and M. Taylor, pers. comm.), and should be finalised in late 2010.

Mulloway

Mulloway, Argyrosomus japonicus (Sciaenidae) are an elusive sportfish in estuarine and coastal waters of southern Australia, southern Africa and China (Silberschneider and Gray, 2008). In Australia, mulloway are distributed around the southern coastline from Bundaberg in Queensland to Exmouth in Western Australia. The species is fast growing, and have a relatively low mortality to growth (M/K) ratio of approximately 0.5 (Silberschneider and Gray, 2005; Taylor et al., 2005). The species is well studied in south-eastern Australia and South Africa, and a stock assessment exists for the species in NSW waters. The maximum reported length for mulloway is 181 cm (42 y old) in South African waters, and 168 cm (23 y old) in Australia (M. Taylor, pers. comm.).

In eastern Australia, sexual maturity is attained at 55 cm TL in males or 68 cm TL in females (Silberschneider and Gray, 2005). Spawning is thought to occur in coastal areas near the mouths of estuaries (Silberschneider and Gray, 2008) during the summer months (Smith, 2003), and pelagic larvae recruit to estuaries to settle in deeper mud and sediment habitats (Neira et al., 1998). Large freshwater inflows into estuaries provide evidence for a relationship between seasonal freshwater pulses and spawning or recruitment events (Ferguson et al., 2008), possibly through both the stimulation of a spawning event and the provision of a low salinity signal which allows fish to locate estuarine habitat. Mulloway juveniles remain dependent on particular riverine habitat such as deep holes (Taylor et al., 2006c), and show a strong association with structures such as estuarine reefs and submerged rock walls. They generally disperse throughout the brackish regions of the estuary (Gray and McDonall, 1993) and favour estuaries with turbid conditions and high seasonal freshwater flows. Mulloway generally favour salinities from <5-25, although larvae grow best at between 5-12.5 (Fielder and Bardsley, 1999) (values proposed under the ANZECC water quality guidelines should be considered the tolerance limits for other water quality parameters). Consequently, juveniles in south-eastern Australia appear to be more prevalent in deeper riverine type estuaries compared to shallow coastal lagoons and estuaries (Silberschneider and Gray 2007). After maturity, fish often undertake coastal migrations, and increasingly make use coastal reef and surf zone habitat. There is a high degree of estuarine residency and site fidelity in the species, with up to 80% of fish returning to former home ranges after their seasonal estuarine egression (Taylor et al., 2006c).

Mulloway are benthic carnivores and undergo substantial ontogenetic changes in diet with increasing size (Taylor et al., 2006b). This coincides with a change in foraging habitat at 50 cm TL, and several abrupt morphological changes, with fish <20 cm TL feeding primarily on shrimp, before shifting to prawns (20-45 cm TL), and then to fish (>45 cm TL). The species has the ability to adapt their diet to the food resources available is different environments, and are effective predators of most crustaceans and small fish (Taylor et al., 2006b).

In NSW, the minimum length for take of A. japonicus is 45 cm. NSW mulloway are considered to be growth overfished, leading to concerns about sustainability of the stock with the bulk of the commercial fishery targeting immature fish (Silberschneider and Gray, 2008). Commercial catches of mulloway in Victorian waters are negligible, but a minimum length for take of mulloway of 60 cm allows some fish to reach sexual maturity before they are taken in the fishery. Australia-wide, mulloway is subject to substantial recreational fishing pressure. An estimated 975 t was harvested by anglers across all states in 2000 (Henry and Lyle, 2003) which far exceeded commercial pressure during the same period (250 t in 2000; Silberschneider and Gray 2008). A detailed description of mulloway biology and fisheries can be found in Griffiths (1996; 1997) for South Africa and Ferguson et al. (2008) and Silberschneider et al. (2009) for southern Australia.

Mulloway are easily cultured at high densities (Fielder and Bardsley, 1999) and several hatcheries in Australia can produce large numbers of fingerlings. There are, however, problems with maintaining the genetic profile of cultured mulloway because of broodstock management issues within hatcheries. Large individuals are expensive to maintain for long periods in the hatchery and take several years to acclimate to the hatchery environment. These problems however, could potentially be circumvented by developing methods to strip spawn wild individuals in the spawning season. A large amount of research has been undertaken into mulloway releases, and techniques exist with which to optimise release densities and strategies for local conditions within target estuaries (Taylor and Suthers, 2008; Taylor et al., 2009).

Black bream

Black bream Acanthopagrus butcheri is an estuarine sparid that is endemic to southern Australia from Myall Lakes in NSW to Kalbari in Western Australia. The species is relatively slow growing in some regions (Morrison et al., 1998), although growth rates appear to be spatially and temporally variable as a result of differences in hydrography, production and habitat (Sarre and Potter, 2000). There is little data from within Victoria with which to assess the M/K ratio, however in Western Australia this can be calculated as 0.59. This is a relatively low value (Taylor et al., 2005), and may not be applicable to Victoria where growth may be slower (Morrison et al., 1998). Black bream reach a maximum length and age of 60 cm and > 30 y and are well studied in Western Australia. Recent research in Victoria has revealed important information on the genetic structure, localised movement patterns and broad habitat associations of adult bream (Burridge and Versace, 2007; Hindell, 2007; Hindell et al., 2008). Some recent information is available on the status of black bream stocks in the Gippsland Lakes, Mallacoota Inlet, Lake Tyers and the Glenelg and Hopkins Rivers, but not for other Victorian estuaries.

On the NSW and Victorian coasts, black bream complete their entire life cycle wholly within estuaries, although there is some exchange between estuaries over small spatial scales (Burridge and Versace, 2007). Male and female black bream reach sexual maturity at 18 cm and 16 cm respectively (Potter et al., 2008), and in smaller riverine estuaries spawning occurs in response to the availability of suitable hydrological conditions during spring or early summer (Newton, 1996). A substantial salt wedge and high degree of stratification is important for bream eggs and larvae, as the halocline at the deeper salt-wedge may act as a barrier which prevents ichthyoplankton from penetrating the outward flowing surface layer and being lost to the sea (Newton, 1996). Moderate salinity and high dissolved oxygen conditions present after flood events also provide conditions favourable for spawning, sufficient egg incubation and hatching, particularly as eggs are sensitive to hypoxia (Hassell et al., 2008a; b). Black bream generally favour temperatures < 25°C and DO > 4 mg L^-1 (Hoeksema et al., 2006), and eggs require a salinity of 15-23 and temperature 16-20°C for successful hatching (Hassell et al., 2008b). Post-larval black bream juveniles are thought to settle into extensive seagrass beds (e.g. Zostera sp.) – and possibly other types of benthic habitat – to provide both a refuge from predation, and a source of food (Butcher, 1945). Adults are euryhaline, preferring the bottom of deep, low salinity pools among wooded debris and beneath riparian vegetation in riverine estuaries. They are more widely distributed in both shallow and deep waters of lagoonal estuaries and on both bare and vegetated substrates. During periods of excessive freshwater flow, adults have been observed departing the estuary (Hindell et al., 2008) for sheltered coastal reefs (Lenanton et al., 1999). Populations in different estuaries are genetically divergent from one another, although there is infrequent gene flow amongst adjacent estuaries (Burridge and Versace, 2007). In NSW, hybidisation with cogeneric yellowfin bream Acanthopagrus australis (Rowland, 1984) has contributed to the loss of 'pure' breed A. butcheri within estuaries.

Larval black bream are planktivorous, feeding predominantly in the lower regions of the water column on calanoid copepods, and spawning times are thought to be timed to coincide with copepod blooms in Victoria's estuaries (Newton, 1996). As postlarval juveniles settle into seagrass beds the diet switches to small invertebrates such as gastropods and polychaetes, and progresses to amphipods, crabs and fish with increasing size (Norriss et al., 2002). Adult fish target a wide range of prey, and juvenile and adult diets vary spatially and seasonally, with fish generally targeting abundant prey rather than feeding selectively (Sarre et al., 2000).

Black bream is one of the most popular angling species in southern Australia, with up to 200 t y^-1 harvested by anglers in Victoria alone (Anon., 2008b). Black bream fisheries in some estuaries have experienced substantial recent declines such as the Blackwood River (Sarre and Potter, 2000) and the Gippsland Lakes, possibly as a result of habitat loss and hydrographic conditions unfavourable for spawning and early survival. The minimum length for take of bream in Victoria is 28 cm, meaning that fish reach sexual maturity and have the opportunity to spawn before entering the fishery. A. butcheri is also an important commercial species in Victoria. The total Victorian bream catch in 2006/07 was 58 tonnes with a wholesale market value of ~$630,000 (Anon., 2008b). Virtually all of this catch comes from the Gippsland Lakes, where the fishery is thought to be fully exploited.

Techniques for rearing black bream have been developed and optimised and the species can be cultured in large numbers at many hatcheries around Australia (Partridge et al., 2003). There are few issues with holding sufficient broodstock to maintain genetic variation (Jenkins et al., 2006), however broodstock should be taken from the target estuary to maintain potential localised genetic variability. Strong estuarine residency make black bream an ideal candidate species as stocked fish are likely to remain in the estuary of release until harvest, although recent acoustic tracking research suggests that inter-estuarine movement of bream may be greater than previously thought (Hindell et al., 2008). Black bream can take up to 9 y to reach the fishery in Victoria, which represents a substantial lag between release and harvest during which natural mortality can occur. Much research has been undertaken on stock enhancement strategies for black bream in Western Australia (Dibden et al., 2000; Jenkins et al., 2006), and coupled with recent research in Victoria's estuaries this provides a solid foundation for the conduct of any future enhancement trials for this species in appropriate Victorian waters.

Estuary perch

Estuary perch Macquaria colonorum (Percichthyidae) are an important angling fish around south-eastern Australia. The species is catadromous, moving between fresh and salt water regions of estuaries (Howell et al., 2005), and residing in most coastal stream systems along the entire Victorian coast. Estuary perch grow fast during early life (Kirwin, 2000), although there are no published mortality estimates. Using the approach of Pauly (1980) and the growth parameters of Kirwin (2000), natural mortality is calculated to be ~0.36 y^-1. This equates to an M/K ratio of 2.58, which is substantially higher than the other teleost species reviewed here and those shown in Taylor et al. (2005). This species is poorly studied throughout south-eastern Australia, and there are no existing stock assessments for the species. Fish have been found to be up to 9 y old, at which point they are ~45 cm FL and ~2 kg in weight (Anon., 2008c). The species can grow up to 10 kg, but such large individuals are extremely rare.

Male and female estuary perch reach maturity at ~22 cm and ~28 cm respectively. Adults move to the mouths of estuaries to breed, although some conflicting evidence suggests spawning occurs in the middle estuarine reaches at salinities >10 (Harris, 1986). Spawning usually occurs during winter (from July to August) when water temperatures are ~15-19 °C, but can span up to 5 months of the year (Newton, 1996). Eggs are laid on submerged rocks, and pelagic larvae are observed in greatest abundance in deep pools (Newton, 1996). Larval estuary perch require ~18°C and salinity of 25-35 (Trnski et al., 2005), and in the absence of any other information values proposed under the ANZECC water quality guidelines should be considered the tolerance limits for other water quality parameters. Juveniles settle out from the larvae at <1 cm SL into beds of aquatic plants such as Zostera sp. (Trnski et al., 2005), which similarly to A. butcheri are important nursery areas providing both food and refuge from predation. Adults are most common in the brackish water of estuaries but are distributed well into freshwater rivers that feed resident estuaries. In Victoria, estuary perch are most abundant in deeper-channeled rivers such as the Glenelg, Hopkins, Curdies and Bemm Rivers, as opposed to shallower coastal lagoons (McCarraher and McKenzie, 1986). Adult estuary perch have occasionally been reported from open coastal waters (McCarraher and McKenzie, 1986).

Along with the congeneric and sympatric Australian bass Macquaria novemaculeata, estuary perch are a prized angling species in the estuaries of south-eastern Australia. Approximately 1-2 t are landed commercially in Victorian waters each year at a value of ~$20,000 (Anon., 2008a). Until recently, few anglers could effectively target estuary perch, and catches were predominantly taken by specialist anglers. Targeted angling for estuary perch has recently increased with the advent of more sophisticated fishing tackle (e.g. soft plastic lures) and boating equipment (e.g. electric motors). Estuary perch grow fastest during their first 3 y and enter the fishery at 27 cm total length, which is greater than the length of maturity for males but not for females. Stocks are considered to be substantially under-exploited (Anon., 2008c), and provide a unique opportunity to expand the fishery, although there are no published estimates of estuary perch catch rates available in primary literature.

Estuary perch display seasonal variation in feeding habitat, concentrating foraging higher in the water column in winter, and on the benthos in summer (Anon., 2008c). Like black bream, larval estuary perch are reliant on copepod nauplii, and the presence of larvae in the water column is timed to overlap with the seasonal peak for important prey (Newton, 1996). Juvenile and adult estuary perch are carnivorous, although their diet varies ontogenetically, spatially and seasonally. In the Hopkins River, juveniles predominantly target the shrimp Paratya australiensis and amphipod Amarinus lacustrine, although the importance of these prey decreased with increasing size (Howell et al., 2005). Adult fish are primarily piscivorous, preying on small yellow-eye mullet and gudgeon (Anon., 2008c). In freshwaters, diet is less diverse and is dominated by larval Tricoptera (Howell et al., 2005).

Estuary perch are abundant in most of the estuaries in which they occur (approximately 78% of Victorian estuaries, McCarraher and McKenzie, 1986), and there are no identified issues with the fishery. The species can be cultured but few, if any facilities produce commercial quantities of the species. The species is yet to be released for the purpose of fishery enhancement, so release strategies will need to be developed. In addition, there is limited knowledge of genetic population structure for estuary perch, although microsatellite markers do exist and could readily be applied in such studies (Schwartz et al., 2005).

Dusky flathead

Dusky flathead, Platycephalus fuscus are an eastern Australian member of Platycephalidae occurring from Cairns in North Queensland to the Gippsland Lakes in Victoria (Gray et al., 2002). Nationally the species is an important target for commercial fisheries, but Victorian catches are small (usually < 40 tonnes), and come almost entirely from the Gippsland Lakes. Flathead are one of Victoria's most popular saltwater angling targets (predominantly dusky and sand flathead, Henry and Lyle, 2003). Dusky flathead are fast growing, but with relatively high mortality (Scandol and Forrest, 2001), resulting in a mortality to growth ratio of ~1.8. P. fuscus grow to ~120 cm, however males are smaller and die earlier (maximum length and age 62 cm and 11 y respectively) than females (maximum length and age 100 cm and 16 y respectively, Gray and Barnes, 2008). The species is relatively poorly studied given its popularity with anglers, however substantial research is currently underway (L. Barnes, pers. comm.). The genetic stock structure of dusky flathead in eastern Australian waters is unknown.

Male and female dusky flathead reach maturity at around 32 cm and 57 cm respectively (Gray and Barnes, 2008). Spawning occurs at the mouths of estuaries and in adjacent coastal areas (Gray and Barnes, 2008) from November to March, with the highest concentration of spawning activity between Ballina and Moreton Bay (West, 1993). Pelagic eggs and larvae disperse along the coast in the East Australian Current, and are present in coastal waters as late as May (Gray and Miskiewicz, 2000). Larvae enter estuaries using the flood tide in a ratchet-like fashion, which allows them to penetrate far enough into the estuary to access key habitat. Dusky flathead likely have a broad salinity tolerance, but in the absence of any other information general values proposed under the ANZECC water quality guidelines should be considered the tolerance limits for other water quality parameters. Juveniles settle into structured habitats including seagrass beds and shallow mangroves (Scandol et al., 2008), although they also utilise bare muddy and sandy habitats (Gray et al., 1996). Adult P. fuscus are concentrated on bare sandy and muddy habitats across a range of depths, in both estuaries and coastal bays, and adults penetrate estuaries as far as the brackish limits. In the Gippsland Lakes, dusky flathead are largely sedentary and remained in single regions for extended periods (Hindell, 2008). As with mulloway, a lagged positive relationship exists between catch per unit effort (CPUE) and the summer freshwater inflow three years prior, which corresponds to the period of time taken to recruit to the fishery (Gillson et al., 2008). The impact of freshwater flow on CPUE is likely acting through recruitment as described previously, either through the initiation of a spawning event or provision or a strong freshwater recruitment signal for coastal pelagic larvae.

P. fuscus uses the bare habitats within which it resides to hide and ambush prey, as it buries into soft sediments. This sit-and-wait strategy means predation occurs non-selectively on passing organisms (e.g. Platell et al., 2006). Prey such as small crabs, shrimp, small fish and polychaete worms, as well as a large amount of organic matter, are common (Hadwen et al., 2007). No studies on the diet of dusky flathead in Victoria's estuaries are currently available in the primary literature.

Dusky flathead represent a small commercial fishery in Victoria (Anon., 2008b) as commercial catches are dominated by other Platycephalidae (Anon., 2008a). Dusky flathead enter the fishery at 27 cm, and can reach this size within 1 y. To protect larger, fecund individuals, daily recreational catch is limited to a single fish over 60 cm TL, however the lower limit of 27 cm does not allow fish to spawn prior to entering the fishery (Gray and Barnes, 2008). Currently, about 35 t are landed annually in Victoria at a value of ~$135,000 (Anon., 2008b). There is limited recent information on the recreational fishery for dusky flathead available in the published literature, however it is thought the recreational fishery for dusky flathead is increasing (Anon., 2008c). The status of dusky flathead stocks in Mallacoota Inlet and Lake Tyers is currently being monitored through information provided by 'research' angler diarists.

Techniques to rear dusky flathead in large numbers for release have been developed previously (Palmer et al., 2000). Whilst several hatcheries in eastern Australia possess the technology to rear the species, it is unknown whether any of these facilities currently maintain broodstock to facilitate this. The enduring drought in south-eastern Australia, and the popularity of large fecund females with recreational anglers, may be contributing to a potential recruitment limitation in the species. Dusky flathead has been released previously in south-eastern Queensland with mixed success, however several techniques important to release were developed during this project (Butcher et al., 2000). The lack of knowledge on genetic stock structure for the species will need to be addressed prior to any trial of fish releases for fisheries enhancement.

Eastern king prawn

Eastern king prawn, Melicertus plebejus is a penaeid prawn occurring from Moreton Bay in central Queensland to eastern Victoria, and the waters of north-eastern Tasmania (Montgomery et al. 2007). Confirmed records of prawns occurring in central Victorian waters between Wilson's Promontory and Cape Otway are rare, and there is some uncertainty regarding the species identity of prawns recorded from Western Port bay and Port Phillip Bay in the early 1980s (C.M. Macdonald and R.H. Winstanley, pers. comm., Montgomery and Winstanley, 1982). Eastern king prawns are fast growing (Taylor, 2008), and experience moderate mortality rates (Glaister et al., 1990; Ives and Scandol, 2007). Assuming an average natural mortality rate of 0.24 y^-1 (Ives and Scandol, 2007), a mortality to growth ratio can be calculated as 0.83. Tagging studies carried out by Montgomery (1990) indicated that eastern king prawns constitute a single unit stock across their geographic range, and preliminary genetic analysis using microsatellite markers supports this (T. Chan, pers. comm.). M. plebejus is very well studied in terms of biology, ecology and fishery in NSW, however few studies exist for Victoria. There are no current stock assessments available for M. plebejus in the published literature, although a joint stock assessment for NSW and Queensland has been proposed (Scandol et al., 2008).

Adult eastern king prawns spawn at ~9 months of age, and spawning is thought to primarily occur off Moreton Bay; although localised spawning events at the mouths of estuaries are a possibility. Spawning occurs in deeper shelf waters (<200 m), and is protracted across several months over the year (Courtney et al., 2002). Females produce >10⁴ planktonic eggs, which hatch into nauplius larvae <1 month after fertilisation. During these stages, eastern king prawn larvae are transported southwards from northern NSW and Queensland via the East Australia Current (EAC, Montgomery, 1990). Post-larvae enter estuaries and settle onto aquatic macrophytes (Ochwada et al., 2009), and also sandy substrates. Prawns rapidly grow to become juveniles, and are then primarily associated with sandy bottoms and less dependent on refuge. Adults also remain present in estuaries almost exclusively over bare habitat (Taylor and Ko, submitted), but may undertake a spawning migration or move into offshore waters (Ruello, 1975). Eastern king prawn have broad salinity tolerances, but in the absence of any other information general values proposed under the ANZECC water quality guidelines should be considered the tolerance limits for other water quality parameters. In coastal lagoons and low flow riverine estuaries, the presence of a sandy berm often traps adults within the estuary, where they can not reproduce and continue to channel energy into mass gain rather than gonadal development (Taylor, 2008). There is a clear link between freshwater flow and eastern king prawn CPUE (Ives and Scandol, 2007), possibly through the stimulation of a spawning or recruitment event.

Eastern king prawns are opportunistic omnivores throughout all life stages. Identification of the food sources of decapod crustaceans is difficult due to the maceration of prey during ingestion (Suthers, 1984), however postlarvae and juveniles are thought to feed on small zooplankton, detritus, microphytobenthos and epiphytic algae. The stomach contents of larger eastern king prawns are more readily identifiable, and show feeding on crustaceans, polychaete worms and bivalves (Suthers, 1984).

The eastern king prawn is a valuable target species that is harvested by commercial fisheries operating in Victoria, NSW and Queensland. In Victoria, total commercial landings are currently ~50 t y^-1, valued at ~$340,000 (Anon., 2008a). There is a small recreational prawn harvest in Victorian waters of around 11 t, which is likely to be predominantly eastern king prawns (based on Henry and Lyle 2003). Recreational fishing for prawns occurs exclusively in shallow estuaries, which allow the effective use of dip nets. Catches of eastern king prawns in Victoria largely depends on the assumption that there is continued and robust recruitment from spawning locations to the north in NSW and Queensland. The Victorian coast is at the southwestern extremity of the distribution of eastern king prawns and away from the main flow of the Eastern Australian Current, which separates from the coast and heads in a south-east direction at Diamond Head, NSW. Both geographical distance and current strength are likely to contribute to the sporadic recruitment of eastern king prawns to estuaries on the eastern Victorian coast, and associated sporadic catches.

Eastern king prawns can easily be cultured to the postlarval and early juvenile stage, however grow out of the species is not financially viable due to the requirement for a high protein diet (which is traditionally expensive). Prawns are generally induced to spawn using eyestalk ablation (Kelemec and Smith, 1980; 1984), and several prawn hatcheries have the capacity to culture the species. Broodstock are easy to collect in large numbers and this can be undertaken on a yearly basis so there is no requirement to maintain spawners over the longer term. Technology for prawn enhancement is well developed for many Penaeidae including the eastern king prawn. Enhancement trials for the eastern kings prawn has been successful in NSW, with recruitment to the fishery within 3 months, >5% survival, and pilot releases of 9 million prawn postlarvae have contributed an estimated 10 t of prawns to the recreational fishery in Wallagoot Lake (Taylor, 2008).

A review of potential opportunities and the need for releases of selected fish species into selected Victorian estuarine waters

Snowy/Brodribb River¹

Estuary

The lower reaches of the Snowy and Brodribb Rivers represent a riverine estuary with substantial expanses of shallow lake and saltmarsh vegetation that supports a small but popular recreational fishery. The Snowy River is listed as a Heritage River Area under the Victorian Heritage Rivers Act (1992). The estuary covers an area of 6.5 km² and drains a catchment of 15,096 km². Much of the catchment consists of sparsely developed agricultural land and national parks, and over time improper land use has led to catchment erosion and sedimentation of the river, which has resulted in a large, semi-permanent sand slug near the town of Orbost. The river has seen a considerable investment by the East Gippsland Catchment Management Authority, which has provided shoreline stablilisation works, weed removal and revegetation, and re-snagging of the river with large woody debris.

Fishery

There is little systematically collected information on fishing activities in the Snowy/Brodribb estuary. Both local East Gippsland residents and tourists angle in the estuary. Commercial fishing is limited to two bait licence holders who operate in the lower estuarine reaches and limited commercial eel fishing conducted via permit in Lake Curlip on the Brodribb River and Lake Corringle on the lower Snowy River. Eastern king prawn, estuary perch, mulloway, black bream and dusky flathead are all historically present and targeted by anglers, and much of the recreational harvest is black bream (Anon., 2008b). Further investigation into the recreational fishery in the estuary would assist in prioritising candidate species for release.

Entrance characteristics and water quality

The Snowy/Brodbibb estuary entrance is intermittently open to the ocean, usually after persistent or heavy rainfall. The estuary is occasionally opened artificially to mitigate flood threats to agricultural land and/or foreshore infrastructure. When inspected for the purposes of this study the estuary entrance was closed and the water column was stratified, with a difference of ~20 between surface and bottom salinity at Lake Curlip, ~9 km upstream of the entrance on the Brodribb River. In the main river channel of the Snowy River, surface salinity was 1 and bottom salinity was 31.5, 8 km upstream of the entrance. All pH (8.3 ± 0.1), DO (6.5 ± 1.4 mg L^-1) and turbidity (7.3 ± 2.4 ntu) measurements were within acceptable levels for fish at all sites sampled.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), from direct observations, and from personal communications with Mr. Dick Brumley and other regional fisheries officers. Estuary perch and dusky flathead were proposed by recreational anglers for release in the Snowy/Brodribb River estuary.

Freshwater inflow and habitat

The Snowy/Brodribb River estuary is fed from a substantial catchment, however upstream water extraction in the Snowy River catchment leads to greatly reduced flows and occasional drying of the river bed in upstream areas. The Snowy River has a high fluvial flow rating, and nutrient loads are almost 10x pre-European settlement yields. Inflow from the Brodribb River and other tributaries to the estuary maintains acceptable estuarine water quality conditions, but current management initiatives aim to increase environmental flows down the Snowy River to 28% of historic mean annual flow (Anon., 2009a). Based on the water quality sampling stations surveyed, the depth of the saline reaches of the estuary is usually <3 m. The extent of seagrass cover in the estuary is unknown, however large woody debris and shoreline stabilisation works have provide structured habitat for resident fishes. There are extensive shallow sand flats at the river mouth, and within Corringle Lake, which connects to the main channel.

Estuary perch

Estuary perch occur in the Snowy/Brodribb River and are a popular angling target species in this estuary. The Snowy/Brodibb River estuary provides ample habitat for estuary perch, and possesses ideal hydrological characteristics for both juveniles and adults. There are no perceived issues with the estuary perch fishery in this estuary, and given appropriate conditions appear to exist for wild spawning, egg/larval survival and settlement of small juveniles, the species is unlikely to be recruitment limited. These factors indicate that there is currently little need for hatchery releases of estuary perch in this system.

Dusky flathead

Dusky flathead are targeted by recreational fishers within the Snowy/Brodribb River estuary. The reduced freshwater flow and increased frequency and persistence of entrance closure of the estuary mean that recruitment of dusky flathead to the estuary from Bass Strait waters is likely to have become more limited in recent years. The extent to which recruitment can be provided from dusky flathead spawning within the estuary is unknown. Extensive sand flats within the estuary provide ideal adult habitat for the species, however the area of seagrass or other suitable habitat to support small juveniles is unknown. Potential recruitment limitation, and an increasing recreational interest in this species mean that dusky flathead may be an appropriate candidate for release into the Snowy/Brodribb River estuary. A more comprehensive survey of seagrass cover or other suitable juvenile dusky flathead habitat in the estuary would assist in making a final determination for this species.

Lake Tyers¹

Estuary

Lake Tyers is a small lagoonal estuary just east of Lakes Entrance. The estuary covers an area of 12 km², and is largely unmodified with a catchment of 570 km² consisting of mainly open forest, and small agricultural and urban settlements. The township of Lake Tyers near the estuary entrance has a small permanent population which increases substantially during traditional tourist seasons. Most of the tourism visits to the estuary are for fishing or other water-based recreational activities.

Fishery

Lake Tyers was declared a Fisheries Reserve in 2004, and commercial fishing has been closed in the estuary since this time. Historically, the commercial fishery in Lake Tyers landed 3 – 48 t y^-1, and principally targeted black bream (Anon., 2007). Luderick Girella tricuspidata, yellow-eye mullet Aldrichetta fosteri and sea mullet Mugil cephalus, and silver trevally Pseudocaranx dentex were also taken commercially within the estuary, as were incidental catches of dusky flathead and estuary perch (Anon., 2007). The only types of commercial fishing permitted in the estuary since 2004 are eel fishing and commercial bait harvesting. Lake Tyers supports a recreational fishery of ~15 t y^-1, dominated by black bream (in the winter) and dusky flathead (in the summer). Eastern king prawns are occasionally captured in good numbers within the estuary, but such events are dependent on successful recruitment of juvenile prawns to the estuary from Bass Strait waters. Successful prawn recruitment events are sporadic due to fluctuations in the strength of the East Australia current supplying larval prawns, and due to intermittent closure of the Lake Tyers estuary entrance. Increased frequency and persistence of entrance closure of the estuary means that prawn recruitment to the estuary is likely to have become more limited in recent years.

Entrance characteristics and water quality

Lake Tyers is intermittently open to the ocean, and opening can occur naturally and mechanically. Mechanical opening is undertaken to mitigate flood threats to agricultural land and/or foreshore infrastructure. The estuary can be closed for very long periods, and this markedly affects the hydrography of the water. Typically, salinity increases and seagrass begins to die-off during periods when the lake is closed. Increasing salinity is thought to have contributed to a recent infestation of resident fish with the parasitic copepod species Caligus epidemicus, as copepod populations proliferate. When inspected for the purposes of this study the estuary had been closed for approximately 9 months. Water quality measurements all fell within the expected range for a healthy estuary, although the salinity was near marine in the upper reaches (32.8 ± 0.2). A thermocline was present across the lower lagoonal reaches of the estuary at a depth of about 1.5 – 2 m, and a halocline was detected just above the bottom in some deeper holes in the Nowa Nowa Arm. This deeper layer of water was also characterised by a high (alkaline) pH.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), the Lake Tyers Fisheries Reserve Management Plan (Anon. 2007), from direct observations, and from personal communications with Dick Brumley and other regional fisheries officers. Mulloway, estuary perch and eastern king prawn were proposed for release in Lake Tyers.

Freshwater inflow and habitat

Lake Tyers is fed by two small creeks, Boggy Creek and Stony Creek, and is classified as having a medium fluvial flow rating. Nutrient and sediment inputs to the estuary are low, and barely exceed pre-European settlement yields. The depth of the lower lagoonal part of the estuary is approximately 3 m with deeper holes in the Nowa Nowa and Toorloo Arms to 24 m, although these holes are patchy and a deep channel is absent from the estuary. The lake contains patchy submerged rocky reef, extensive saltmarsh, and substantial riparian vegetation and snags in the upper reaches. The cover of seagrass fluctuates annually and seasonally, however patches are estimated to cover >0.5 km². There are extensive shallow sand flats within the estuary, and on inspection these appeared to have a good cover of microphytobenthos.

Mulloway

Larger mulloway have been captured in the estuary, but there are no records of juveniles occurring here. This is not surprising, as mulloway traditionally recruit to riverine estuaries as juveniles, and rely on substantial areas of deeper, structured habitat (Taylor et al., 2006c). Whilst the presence of a sandy berm at the estuary entrance and low freshwater input could contribute to limitation of mulloway recruitment from Bass Strait waters, the lack of key juvenile habitat within the estuary means that mulloway releases are unlikely to improve recruitment and should not be undertaken in this estuary. This is further supported by previous stocking research which has indicated high mortality and low success of mulloway releases into estuaries with lagoonal profiles (Taylor et al., 2009).

Estuary perch

Estuary perch occur in Lake Tyers, and are taken in low numbers in the recreational fishery. Estuary perch prefer deeper-channeled riverine estuaries, and this type of habitat is limited in Lake Tyers, although there is a good amount of riparian vegetation and snags in the upper reaches of the two arms of the estuary. The lack of rocky substrate or other suitable juvenile habitat may contribute to a recruitment limitation in the species, and the estuary could possibly support a small release of estuary perch to circumvent this limitation. These considerations, together with the current small interest in estuary perch by anglers fishing in Lake Tyers, mean that the costs and benefits of estuary perch releases would need to be carefully assessed, and may not be justifiable.

Eastern king prawn

The eastern king prawn fishery in Lake Tyers fluctuates substantially as a direct result of periodic limitation of prawn recruitment from Bass Strait waters due to closure of the estuary entrance. Released prawns grow well within NSW estuaries where there are abundant habitat and food resources within the estuary to support the population. Lake Tyers appears to have abundant food and habitat resources, however this should be further investigated in a quantitative fashion. There is also substantial interest in recreational prawn fishing, and the bathymetry of the lagoonal part of the estuary is suitable to recreational prawning. Given these factors and the existence of technology to rear and release the species, eastern king prawns would make an ideal candidate for release in Lake Tyers, and releases would be most ideal when the estuary has remained closed during the months of spring and early summer, or when the EAC is weak.

Non-identified species

Recruitment of dusky flathead to Lake Tyers from Bass Strait waters is potentially limited because of entrance closures, although there is some evidence that the species has reproduced within the estuary (Anon., 2007). Potential external recruitment limitation, the presence of suitable seagrass and sandy habitat, and strong recreational interest in this species, means that dusky flathead may be a suitable candidate for release into this estuary.

Anderson Inlet¹

Estuary

Anderson Inlet is a substantial estuary with about 2.5 km² of open water and 13 km² of tidal sand flats in the lower lagoonal section. The estuary is located in South Gippsland adjacent to the town of Inverloch, and freshwater inflow is primarily from the Tarwin River, which drains ~1,715 km² of mainly agricultural land and enters at the south-eastern corner of the inlet. Approximately 3 km² of the lower lagoonal part of the estuary is covered by the invasive tussock grass Spartina, which is thought to have contributed to a decline in seagrass cover within the estuary (Anon., 2006a). Anderson Inlet is a popular tourist destination particularly with people from Melbourne, who come to the region for fishing and other water-based recreational activities.

Fishery

Anderson Inlet was closed to commercial scalefish fishing in 2000 and was declared a Fisheries Reserve in 2003. The only commercial fishing activities still permitted in the estuary are eel fishing and commercial bait harvesting. The estuary supports a substantial year-round fishery for resident Gippsland anglers, with large influxes of visiting anglers during peak holiday periods. King George whiting, sand flathead and Australian salmon dominate the recreational catch in the estuary. Anglers also target estuary perch, silver trevally, yellow-eye mullet and gummy shark. Black bream are captured in low numbers within the estuary.

Entrance characteristics and water quality

Anderson Inlet is permanently open to the sea, and has a restricted entrance that is characterised by a deeper channel that flows into an extensive delta. Considering the high tidal exchange the water quality in the lower lagoonal part of the estuary is largely characterised by prevailing coastal oceanography. Water quality and sediment characteristics in the upper lagoonal and riverine parts of the estuary are more strongly influenced by freshwater flows and fluvial processes. When inspected for the purposes of this study the water in the lower lagoonal part of the estuary was typical of near-marine conditions, with all water quality parameters within the acceptable range for marine and estuarine fishes (salinity 32.3 ± 1.6; pH 8.3 ± 0.1 ; DO 6.4 ± 2.2 mg L^-1; turbidity 9.2 ± 1.6 ntu). There was no boat access provided during this visit, so stratification in the water column was not assessed. The water quality of the Tarwin River was substantially different to the lower lagoonal part of the estuary, with high turbidity, low salinity and low dissolved oxygen.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), the Anderson Inlet Fisheries Reserve Management Plan (Anon. 2006a), and from direct observations. No regional fisheries officers were available for consultation at the estuary. Black bream were proposed for release in Anderson Inlet.

Freshwater inflow and habitat

Anderson Inlet has a high fluvial flow rating, with most of the freshwater entering the estuary through the Tarwin River. Nitrogen loads, phosphorus loads and fine sediment input to the estuary is 5x, 6x and 10x pre-European settlement yields, the latter being evident in high turbidity readings in the Tarwin River. Anderson Inlet has ~1 km² of subtidal aquatic vegetation including ~0.5 km² of Zostera beds (Blake et al., 2000), 0.95 km² of mangroves, and extensive shallow sand flats, however no rocky reef is present within the inlet.

Black bream

Black bream represent a relatively small component of the fishery. Given the broad popularity of bream as a recreational target species, low catches of this species in Anderson Inlet are more likely to be due to low abundance arising from recruitment limitation rather than lack of angler interest. High salinity within the lagoonal part of the estuary, together with limited suitable habitat for both small juvenile and adult bream, mean that conditions may not be optimal for black bream production in this estuary. On the basis of this information, releases of black bream in Anderson Inlet are not recommended. However, if a more comprehensive survey of habitat within the estuary indicates underutilised carrying capacity for bream, then the suitability of this species for release may need to be revised.

Non-identified species

Flathead (mainly sand flathead Platycephalus bassensis) are thought to be the most heavily targeted species in Anderson Inlet (Anon., 2006a). The bay is open to the ocean and has a high fluvial flow so recruitment limitation from hydrological factors is unlikely, however during periods of low flow (such as during drought) recruitment limitation may occur. Key habitat is available in the lower lagoonal reaches of the estuary to support this essentially marine species, and given the high fishing pressure on the species releases of sand flathead fingerlings may make use of surplus productivity released by the recreational harvest. An up-todate assessment of the population biomass and recreational harvest of sand flathead is required to better identify enhancement opportunities for this species in Anderson Inlet.

Patterson River¹

Estuary

The Patterson River estuary is a short, heavily urbanised estuary on the eastern shore of Port Philip Bay. Both the estuary and river drain a catchment of 684 km² into a small waterway area of 0.5 km², resulting in a high fluvial flow classification. The catchment is extensively developed with urban housing. This includes Patterson Lakes, the largest marina development in Victoria, which joins the Patterson River ~3 km inland, and is delimited from the river channel by floodgates. The estuary has a high recreational amenity with four boat ramps along its length of ~4 km, and is a major access point for Port Philip Bay.

Fishery

There is no available information on the fishery of the Patterson River estuary, however anecdotal reports indicate that black bream and mulloway are targeted in the estuary. Other species caught include Australian salmon, trevally, mullet, flathead and short-finned eels. There is no commercial fishing within this waterway, but illegal netting does occur (Anon., 2008b). Due to the potential number of anglers accessing the estuary, there is a potential for considerable fishing effort on these species, however further information is required to ascertain whether this is the case.

Entrance characteristics and water quality

The Patterson River estuary is permanently open to the marine waters of Port Phillip Bay, with training breakwalls constructed on either side of the river mouth. Considering the short length of the estuary (5.5 km) and benign bathymetry, hydrography is likely determined by prevailing conditions within Port Philip Bay, with the exception of periods of inundation. The visit to the estuary occurred directly after a night of moderate rainfall, and water quality parameters were highly variable although still within the acceptable range for marine and estuarine fishes (salinity 9.4 ± 6.6; pH 8.5 ± 0.1 ; DO 5.3 ±

0.6 mg L^-1; turbidity 49.6 ± 17.8 ntu). The layer of fresh water at the surface was <1 m deep. Rocky rapids and a ~100 m long fishway delimit the tidal and freshwater sections of the river, and at the limit of tidal penetration the salinity was as low as 1.2.

Freshwater inflow and habitat

The Patterson River experiences substantial freshwater inflow relative to waterway area, and this brings fine sediment and nitrogen inputs to the estuary as high as 20x pre-European settlement yields. The river has no aquatic macrophyte habitat, although the banks along the majority of the river are reinforced with rock walls, which provides good artificial rocky reef habitat to support marine fishes. This short estuary consists primarily of a single unbranched channel with a soft sediment substrate, providing a relatively homogenous habitat profile.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), and from direct observations. No regional fisheries officers were available for consultation at the estuary at the time of the visit, and as such the estuary was surveyed from the shore only. Regional fisheries officers Joshua Hannaford and Evan Coker provided some advice on the fishery in the Patterson River at a later time. Mulloway and black bream were proposed for release in the Patterson River.

Mulloway and black bream

High fluvial flow, occasional stratification, and a permanent opening to the ocean mean that mulloway and black bream are unlikely to be recruitment limited in the Patterson River. Therefore, releases of these species are not recommended at this location.

Werribee River¹

Estuary

The Werribee River estuary is a moderately urbanised estuary on the north-western shore of Port Philip Bay. Both the river and estuary drain a catchment of 1,484 km² of primarily agricultural and increasingly urbanised catchment into a small waterway area of 0.5 km². The estuarine part of the river is highly modified, with shoreline erosion due to the encroachment of agriculture toward the river, and the estuary also drains a golf course and sewerage treatment plant on the southern shore. The presence of good numbers of waterbirds on the western side of the river has seen the estuary declared as a Ramsar listed wetland.

Fishery

There is no available information on the fishery of the Werribee River estuary. Black bream are the main angling target and there is a prime fishery for this species, and mulloway are rarely landed. Estuary perch have been detected within the Werribee River previously, but at present the species is not caught in any numbers. There is no commercial fishing within this waterway (Anon., 2008b), however illegal netting does occur and can lead to removal of substantial numbers of spawning adult black bream.

Entrance characteristics and water quality

The Werribee River estuary is permanently open to the marine waters of Port Phillip Bay, although a sandy berm partially restricts the entrance. The estuary is short (6 km) with a shallow bathymetry, and the upper brackish area of the river is delimited with a rocky barrier (Sherwood et al., 2005) which retains water for irrigation of the adjacent golf course. The visit to the estuary occurred directly after a moderate downpour, and only the surface layer was sampled from the shore as a boat could not be accessed for this estuary. Water quality parameters were highly variable, and some parameters (DO and pH) were at the limit of the acceptable range for marine and estuarine fishes (salinity 19.6 ± 9.5; pH

9.0 ± 0.4 ; DO 2.5 ± 0.9 mg L^-1; turbidity 6.0 ± 1.4 ntu). Water was hypoxic at the water quality station sampled upstream of the weir.

Freshwater inflow and habitat

The Werribee River drains a substantial catchment and has a high fluvial flow rating. Extensive agricultural land use in the catchment has led to fine sediment phosphorus and nitrogen loads that are ~5x pre-European settlement yields. There is no aquatic macrophyte in the river itself, however seagrass beds were observed in Port Philip Bay directly adjacent to the river mouth. Three deep pools have been observed, 3.5 km, 4.2 km and 5.8 km upstream of the mouth of the river (Sherwood et al., 2005). There is substantial riparian vegetation in the upper reaches of the river (upstream of the weir), however there is no facility for fish passage across this barrier during periods of low flow, and during periods of high flow fish may be unable to navigate this barrier in the face of substantial currents.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), and from direct observations. No regional fisheries officers were available for consultation at the estuary at the time of the visit, and as such the estuary was surveyed from the shore only. Regional fisheries officers Joshua Hannaford and Evan Coker were consulted on site, after the river had been assessed. Mulloway and estuary perch were proposed for release in the Werribee River.

Mulloway

There is no evidence that mulloway have ever been captured in the Werribee River, and this is probably a reflection of the poor water quality and lack of deep hole habitat. The mouth of the estuary is permanently open, meaning that wild recruits could penetrate the system if present in that region of Port Phillip Bay, and as such the species is probably not recruitment limited. The combination of the lack of key juvenile habitat and the absence of this species historically in the Werribee River, mean that mulloway releases should not be undertaken within the estuary.

Estuary perch

Estuary perch, while historically present in the river, are not usually captured by anglers here. The low abundance and current perceived absence of the species may be due in part to the lack of both deep pools and riparian vegetation in the brackish section of the river, however ample aquatic macrophytes are available at the mouth of the river to support juveniles. Considering these factors alongside the water quality issues within the river, and the general good health of estuary perch stocks across Victoria, stocking of estuary perch is not recommended within the Werribee River.

Anglesea River¹

Estuary

The Anglesea River estuary is a small intermittently open estuary of 0.05 km² waterway area. The estuary includes Coogoorah Park, which has been developed with amenities for recreational fishing and other activities, and the park includes ~0.16 km² of intertidal sandflats. The main channel of the estuary is dredged to approximately 1 km from the mouth. The 125 km² catchment lies within the Otway basin, and is mainly comprised of forest, with a small urban settlement and the Alcoa Coal Mine. The estuarine section of the Anglesea River extends to Coal Mine Road, where a weir prevents further intrusion of saline water.

Fishery

There is limited information on the fishery in the Anglesea River. Estuary perch, yellow-eye mullet, shortfin eel Anguilla australis, silver trevally, Australian salmon, sea mullet Mugil cephalus and black bream are known to occur in the estuary. Estuary perch are captured in low numbers, and the bulk of the recreational fishery targets black bream. Anecdotal information from the recreational fishery indicates that the black bream are small and stunted, meaning that food resources may be limited within the estuary.

Entrance characteristics and water quality

The estuary is often closed to the sea, and most openings are artificial. Artificial opening is the responsibility of the Surf Coast Shire and is undertaken with Coastal Management Act consent under a permit from the Department of Sustainability & Environment and a works-on-waterways permit for the Corangamite Catchment Management Authority (CCMA). Recently, the CCMA has introduced the Estuary Entrance Management Support System (EEMSS) to assist the Shire in decisions on estuary openings. Due to the long periods of time the estuary spends closed to the ocean and the presence of sulphidic soils in the catchment, water quality issues frequently occur in the estuary. The most severe of these is low pH (Anon., 2005), which contributes to fish kills every few years. The estuary was visited during a substantial downpour, and water quality measurements reflected this (salinity 4.0 ± 0.1; pH 10.5 ± 0.6 ; DO 3.9 ± 0.3 mg L^-1; turbidity 8.8 ± 0.6 ntu). A highly alkaline pH was surprising, and contrasts with the lower (acidic) pH values the estuary often experiences. A low DO value is characteristic of the estuary.

Freshwater inflow and habitat

The Anglesea River receives relatively low natural freshwater inputs, however the Alcoa Coal Mine does discharge a substantial amount of water to the estuary. Nutrient inputs barely exceed pre-European settlement yields. The estuary is shallow, and does not include a deep, well-defined channel. Approximately 10% of the estuary is covered in Zostera muelleri and Ruppia megacarpa, and macrophytes cover ~50% of the estuary in total. The estuary possesses ~0.03 km² of rocky substrate.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), the Anglesea Estuary Management Plan (Anon. 2005b), from direct observations, and from personal communications with Peter Lawson and other regional fisheries officers. Estuary perch and eastern king prawn were proposed for release in the Anglesea River.

Eastern king prawn

The Anglesea River is outside the natural range of any estuarine prawn, and there is no published evidence to suggest that prawns have occurred in the river historically. Releases of eastern king prawn are not recommended in this estuary.

Estuary perch

Estuary perch are present in low numbers within the river, and are targeted by recreational fishers. The water is sufficiently brackish to support the adults, and there is no evidence to suggest recruitment limitation. The lack of deeper water within the estuary and high abundance of black bream probably contributes to low numbers of the species. Considering these factors and water quality issues, the Angelsea River does not provide a good opportunity to support releases of hatchery-reared estuary perch relative to other estuaries.

Painkalac Creek¹

Estuary

Painkalac Creek and Aireys Inlet represent a small, shallow unmodified estuary, with a waterway area of 0.16 km². This estuary was measured to be ~4 km long, and drains a catchment of mostly forest, with limited urban and agricultural land use.

Fishery

There is no information on the fishery in Painkalac Creek. Short-finned eel, sea and yellow-eye mullet, black bream, luderick, estuary perch, long-snouted flounder Engyprosopon rostratum and Australian salmon have all been detected in the estuary, however the extent to which these species are targeted by the recreational fishery is unknown. Anecdotal information suggests that estuary perch are most popular with anglers in this estuary, followed by black bream.

Entrance characteristics and water quality

The estuary is predominantly closed to the sea, due to low freshwater inflow and an extensive sandy berm ~0.2 km wide. The estuary is artificially opened using the protocols described for the Anglesea River, usually in response to threats of flooding to shoreline infrastructure. Water quality measured at the time of the visit was characteristic of normal conditions with stratification of the top 0.5 m, and fell within the acceptable range for marine and estuarine fishes with the exception of DO (salinity 14.3 ± 3.4; pH 8.6 ± 0.3; DO 2.4 ± 0.1 mg L^-1; turbidity 15.4 ± 8.0 ntu). Dissolved oxygen is usually within acceptable levels in Painkalac Creek, and the factors contributing to the low reading at the time of the visit are unknown.

Freshwater inflow and habitat

Painkalac Creek receives relatively low freshwater inputs due to a 409 mL capacity dam situated ~6.5 km upstream from the mouth. There is no available data on nutrient inputs to the estuary. The estuary is shallow, contains 0.1 km² of intertidal sand flats and saltmarsh, and ~50% of the estuary is covered in beds of Zostera sp. and Ruppia megacarpa.

Estuary perch

Estuary perch are thought to be the most popular angling target within the estuary. The waterway is brackish for most of its extent, and possesses habitats necessary for estuary perch including seagrass and riparian vegetation. The lack of rocky substrates on which to lay eggs may contribute to a recruitment limitation, although the use of other substrates within the estuary for this should be investigated (such as vegetated soft bottoms). Based on these factors, Painkalac Creek appears to be an appropriate estuary in which to undertake a trial release of estuary perch fingerlings. The costs and benefits of releases, however, should be carefully considered in the context of the status of the fishery in this waterway.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), from direct observations, and from personal communications with Peter Lawson and other regional fisheries officers. Estuary perch were proposed for release in the Painkalac Creek.

Barham River¹

Estuary

The Barham River estuary is a shallow (3 m maximum depth) modified coastal river system adjacent to the community of Apollo Bay. The lower catchment of the river system is primarily native forest, with ~15% agricultural land use.

Fishery

There is no published information available regarding the recreational catch of marine and estuarine species in the Barham River. Potential recreational species occurring within the estuary include estuary perch, black bream, Australian herring (tommy ruff) Arripis georgianus, silver trevally, Australian salmon, sea and yellow eyed mullet, and greenback flounder Rhombosolea tapirina. Brown trout are present at high densities in the upper reaches of the river. Anecdotal information suggests that black bream and estuary perch are the most common targets of anglers in the estuarine parts of the river.

Entrance characteristics and water quality

The estuary is predominantly closed to the sea, and has recently been equipped with fishing platforms in the lower reaches. The visit to the estuary occurred following a moderate rainfall event, and a shallow freshwater lens was present at the surface of the estuary. Water quality measurements were within the acceptable range for marine and estuarine fishes with the exception of DO (salinity 3.5 ± 3.2; pH 8.6 ± 0.2; DO 3.3 ± 0.7 mg L^-1; turbidity 15.4 ± 8.0 ntu). Dissolved oxygen tended to be lower at depth, as is characteristic of the Barham River.

Freshwater inflow and habitat

The Barham River receives relatively low freshwater inflows, and this is further reduced through diversion for agricultural and domestic use. There are no barriers to inflow, and nutrient input is approximately equivalent to pre-European settlement yields. The estuarine section of the river lacks substantial riparian vegetation, although minor re-snagging work has occurred in the vicinity of the fishing platforms to create more structured habitat. There is minimal macrophyte cover (0.007 km²), which is mainly Zostera sp., and no rocky substrates.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), from direct observations, and from personal communications with Paul Millar and other regional fisheries officers. Estuary perch and black bream were proposed for release in the Barham River.

Estuary perch and black bream

Estuary perch and black bream are present in low numbers within the Barham River. The lack of reef, riparian vegetation, suitable juvenile habitat or large woody debris in the estuarine section of the river, however, indicates release of these species may not be successful, and therefore are not recommended.

Curdies Inlet and Curdies River¹

Estuary

Curdies River/Curdies Inlet is an intermittently open estuary that is comprised of a small narrow river system that drains to a large lagoon near the mouth. The estuary has a total waterway area of ~3 km², including ~0.25 km² of intertidal sandflats. The system drains a catchment of ~1,006 km², which includes mostly agricultural land. Curdies Inlet is situated adjacent to the town of Peterborough.

Fishery

There is no published information available on the fishery in Curdies River and Curdies Inlet. The fishery is recreational only, with the exception of a commercial eel harvest. Short-finned eel, sea and yellow-eye mullet, black bream, estuary perch, mulloway and sand flathead Platycephalus bassensis all occur in the estuary. The catch of black bream dominates the fishery, however the extent to which other species are targeted is unknown.

Entrance characteristics and water quality

Curdies Inlet is often closed to the sea. The estuary is artificially opened in response to threats of flooding of low lying agricultural land under the protocols described for other CCMA estuaries above. At the time of the visit the water in Curdies River was highly stratified, consisting of a relatively fresh upper layer of acceptable water quality but low DO; and an anoxic and highly alkaline layer at depth (salinity 9.5 ± 2.9; pH 11.2 ± 0.4 ; DO 1.8 ± 0.8 mg L^-1; turbidity 2.9 ± 0.5 ntu). This contrasted with measurements in Curdies Inlet, which was well-mixed, clear marine water with high DO. Curdies River flows into Curdies Inlet through a narrow mouth which contains a permanent sandbank. Considering the shallow depth of Curdies Inlet (~1 m) relative to Curdies River (6 m), the saline lens likely represents water that has been trapped after the last period of tidal exchange, and has remained static within the deeper river channel since. This anoxic layer is likely to remain in place in lieu of significant water movement in the river (i.e. tidal mixing or a strong freshwater pulse).

Freshwater inflow and habitat

Curdies River/Curdies Inlet is classified as having a moderate fluvial flow. There are elevated nutrient and sediment levels relative to pre-European settlement yields, and current fine sediment input is ~50x pre-settlement levels. Curdies Inlet contains substantial shallow sand/mud flats, including ~0.35 km² of seagrass. The lower reaches of the river and inlet lack good riparian vegetation, but the upstream shoreline is well vegetated.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), from direct observations, and from personal communications with Stephen Eddy and other regional fisheries officers. Mulloway were proposed for release in the Curdies River.

Mulloway

Curdies River was identified as a potential site for mulloway releases, however our data indicates a lack of habitat to support mulloway, particularly in the brackish regions of the river. Mulloway, especially during the juvenile stage, are a demersal species (Taylor et al., 2006a), but conditions in the Curdies River at present prevent access to benthic habitats. Whilst released fish would likely avoid the anoxic water, the inability to access key habitat will likely result in reduced survival (Taylor et al., 2009). Given low numbers of mulloway observed, and the water quality problems in the river, release of mulloway is not recommended.

Non-identified species

Given the large expanse of sand flats in Curdies Inlet, sand flathead (Platycephalus bassensis) are a potential candidate species for this estuary. The frequent closure of the estuary to the sea could present a limitation to recruitment for the species, however more information is required on spawning outside Port Philip Bay. The potential for releases of sand flathead should also be gauged against interest from the fishery.

Hopkins River¹

Estuary

The Hopkins River is an extensive coastal river system ~240 km in length, extending as far inland as Ararat. The Hopkins River estuary extends as far inland as Tooram Stones at Allansford, 8.8 km from the mouth. The river and estuary drains a catchment of 8,984 km² into an estuarine area of ~1.79 km². The river enters the ocean at Warrnambool, however the entrance is closed most of the time. The estuary is highly modified and in relatively poor condition due to poor land use practices in the catchment and freshwater extraction.

Fishery

There are no commercial licenses for eel harvesting in the Hopkins River (Anon., 2006b), however commercial bait harvesting is undertaken. There is no published quantitative information available on the recreational fishery in the Hopkins River, although recreational fishing effort is thought to be 'medium' in the lower reaches The estuary supports popular recreational fisheries for estuary perch and black bream, with other important species including mulloway, Australian salmon and yellow-eye mullet. The Hopkins River contains a healthy population of shortfinned eel, and the juvenile migration over the Tooram Stones is the focus of substantial tourist interest.

Entrance characteristics and water quality

Excessive freshwater extraction means that the entrance to the Hopkins River estuary is closed more often than it is open. The estuary is mechanically opened in response to threats of flooding of shoreline infrastructure, under the Estuary Entrance Management Support System (EEMSS). At the time of the visit the water in the Hopkins River was highly stratified. Water quality was acceptable in the lower reaches of the estuary, however the upper reaches were characterised by low DO (salinity 20.7 ± 4.0; DO 3.3 ± 1.0 mg L^-1; turbidity 11.0 ± 4.0 ntu).

Freshwater inflow and habitat

The Hopkins River is classified as having a high fluvial flow rating, however freshwater flows have become depressed in the ongoing drought. Fine sediment and nitrogen yields in the estuary are several orders of magnitude higher than pre-European settlement yields. The estuarine reaches of the Hopkins River have scattered seagrass patches, however riparian vegetation has been lost from the shoreline along the length of the estuary. The estuary contains substantial rocky substrates and submerged rock walls, including 1.33 km² of river channel.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), the Glenelg Hopkins Fisheries Management Plan (Anon. 2006b), the Hopkins Estuary Management Plan (Anon. 2005a), from direct observations, and from personal communications with Ian Westhorpe and Marcus Clarke. Mulloway and black bream were proposed for release in the Barham River.

Mulloway

Mulloway were identified as a potential candidate species for release into the Hopkins River. There is strong evidence for recruitment limitation due to entrance closures and lack of access from the sea, and habitat is available within the estuary to support the species. There is recreational interest in targeting mulloway in the Hopkins River, and coupled with the above factors this warrants further investigation of stocking potential in this estuary. Mulloway grow quickly to become top-level predators and will likely prey on juvenile eels and bream within the estuary after they reach 3-4 years of age. Given the significance of the eel population, the potential adverse predatory affects of released mulloway on these organisms needs to be considered, and conservative densities released if a trial is undertaken. Also, the vulnerable Australian grayling Prototroctes maraena occurs within the Hopkins River. Larger piscivorous mulloway could potentially predate on this species, although their diet mainly includes schooling fish such as estuarine clupeids (Taylor et al., 2006b). The low abundance of grayling, and feeding habits of mulloway mean that whilst predation on this species by stocked fish could occur, predation is unlikely.

Black bream

Black bream were also identified by stakeholders as desirable for release into the Hopkins River. Given that a good salt wedge exists in the estuary, and there are no perceived problems with the fishery, release of this species is probably not necessary.

Merri River¹

Estuary

The Merri River is a small, extensively modified, river system draining a primarily agricultural and urban catchment. The river has two entrances to the ocean; a natural and man-made entrance. Both entrances flow into the Merri Marine Sanctuary at Warrnambool.

Fishery

There is minimal recreational fishing for any species identified for appraisal in this study. The Merri River fishery is primarily concentrated on stocked brown trout, with black bream, Australian salmon and mulloway taken occasionally. A commercial eel fishery also exists in this river system.

Entrance characteristics and water quality

The man made entrance to the Merri River is permanently open, and the natural entrance opens about twice a year. There is no available fluvial flow classification, however excessive suspended sediment inputs into the estuary have elevated nitrogen yields to ~70x pre-European settlement values. At the time the estuary was visited, water quality in the man-made channel was fresh, with moderate DO, with salinity ~1 less than 1 km from the entrance. At the mouth of the natural entrance, there was a small lagoon in which the water quality was characteristic of marine water.

Freshwater inflow and habitat

The fluvial flow of the Merri River has not been classified, however flow is greatly reduced relative to historic levels. There are small patches of Ruppia sp. in the lagoon adjacent to the natural opening to the ocean, and algal macrophytes are present in the upper estuarine reaches. Small patches of saltmarsh are present throughout the estuary, however there is minimal rocky reef and no deep-holes or pools.

Mulloway

Mulloway were identified as a potential candidate species for release into the Merri River. As the entrance is permanently open the probability of recruitment limitation is reduced, but reduced freshwater inflow is also of concern. Mulloway are not traditionally targeted in this estuary, and there is almost a complete lack of juvenile and adult habitat to support the species. The lack of habitat and low recreational fishing pressure mean that the Merri River is not an ideal estuary for mulloway releases.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), the Glenelg Hopkins Fisheries Management Plan (Anon. 2006b), the Merri Estuary Management Plan (Anon. 2008e), from direct observations, and from personal communications with Ian Westhorpe and Marcus Clarke. Mulloway were proposed for release in the Merri River.

Lake Yambuk¹

Estuary

Lake Yambuk is the lagoonal part of the Eumeralla River estuary. The Eumeralla River is a small, largely unmodified river system draining a 1,022 km² catchment of mainly agricultural land with minor patches of natural forest. Lake Yambuk has a waterway area of 1.1 km², and is adjoined by a small number of dwellings and a small caravan park, which sees a seasonal influx of tourists. The lake itself and the adjoining property of Deen Maar are of national significance, due to their respective ecological and cultural value.

Fishery

Little information is available for the fishery in Lake Yambuk. There is a minor commercial eel fishery in the estuary, and the most common species of fish captured recreationally include black bream, yellow-eye mullet and Australia salmon. Mulloway are also occasionally found in Lake Yambuk.

Entrance characteristics and water quality

There is a single entrance to Lake Yambuk which mostly remains closed to the sea. Most of the occasional estuary openings are natural following strong freshwater flows, but some artificial openings are performed in response to threats of flooding of agricultural land, under the Estuary Entrance Management Support System (EEMSS). Water quality measurements taken at the time of the visit were characteristic for Lake Yambuk, although low dissolved oxygen levels were recorded (salinity 10.9 ± 0.9; DO 3.0 ± 0.1 mg L^-1; turbidity 3.0 ± 2.4 ntu). Water quality in the lake is highly variable and occasionally deteriorates, leading to ulcerations and disease in stressed fish.

Freshwater inflow and habitat

Lake Yambuk has a medium fluvial flow rating, and nutrient yields are only slightly elevated relative to pre-European values. Aquatic macrophytes in Lake Yambuk include the seagrasses Ruppia maritima and Zostera sp., the common reed (Phragmites australis), and other bulrush and sedge varieties. The coverage of seagrass is currently unknown, however there is 2.8 km² of saltmarsh adjoining the lake.

¹ Unless otherwise indicated, information presented here was gathered from the OzCoasts database (http://www.ozcoasts.org.au/), the Glenelg Hopkins Fisheries Management Plan (Anon. 2006b), the Yambuk Lake and Wetlands Management Plan (Anon. 2004), from direct observations, and from personal communications with regional fisheries officers. Western king prawn were proposed for release in the Yambak Lake.

Western king prawn

Whilst habitat is available to support prawn species within Lake Yambuk, this estuary is outside the natural geographic range of the species. There are no estuarine species of prawn that occur in this region of Australia.

Summary and recommendations for further research

Twelve estuaries were evaluated for stock enhancement potential on the basis of potential recruitment limitation, habitat availability, estuarine hydrology, and fishery characteristics. Of the 20 estuary – species combinations investigated, several opportunities that warrant further investigation were identified. These are summarised in Table 2. Several areas for further research are also outlined in Table 2, and these represent unknown aspects of the biology/ecology of the target species, or the estuary or fishery. Assessing these factors are not only important for the design of effective release programs, but are also directly relevant to understanding the dynamics of Victoria's estuarine/marine ecosystems and the management of Victoria's fisheries.

Notwithstanding a full Environmental Impact and Risk Assessment of the estuary – species combinations identified in Table 2, some additional research is required prior to trial releases for any species. Estimated costs associated with this research, as well as costs of fingerlings, are included in Table 3. Items that require further scientific investigation include:

1 Detailed mapping of key habitat is required for all estuaries in which fish/prawns will be released. This would be included in the Environmental Impact Assessment described in the following section

• This will allow optimisation and targeting of fish releases to maximise survival of released postlarvae/fingerlings;
• This will also result in habitat maps which anglers can use to target the released species in stocked estuaries;

2 Development of marking techniques for all fish/prawns species to be released. This is in addition to the Environmental Impact and Risk Assessment described in the following section

• Calcein staining is now the national benchmark for marking fish for release (Crook et al., 2009), and is the only such chemical that may be used for fish marking, and approval is given on a case-by-case basis;
• The osmotic induction technique could readily be applied to the species detailed here with some optimisation of immersion conditions through trials using small numbers of fish;

3 Assessment of release density for each release to be undertaken. This would be included in the Environmental Impact and Risk Assessment described in the following section

• This can be achieved through predatory impact modelling to estimate stocking density on the basis of food and habitat availability;
• Predatory impact modelling allows the manager to assign a proportion of productivity, which aligns with the management strategies for the estuary and fishery (e.g. 1%, 5%, 10% of productivity etc). The predatory impact model determines a stocking density that minimises the probability that predatory impact will exceed the assigned level of productivity;
• This approach essentially reverses the traditional process of releasing fish and then assessing the release: rather than releasing an arbitrary number of fish and assessing the predatory impact, an a priori acceptable predatory impact is used to determine the release density;
• The current format of the Generalised Predatory Impact Model is currently being prepared for publication (Taylor et al., In preparation), however some components of the concept are described in (Taylor and Suthers, 2008);
• Application of this approach will also provide estimates of predation, productivity of the released fish, and potential harvest by recreational anglers.

Table 2. Summary list of best estuary – species combinations recommended on the basis of potential recruitment limitation, habitat availability, estuarine hydrology and fishery characteristics. Some key knowledge gaps that should be addressed during the development of any release program are also indicated.

¹ From Kramer (2009)
² NOTE: These are applicable to species and need not be addressed for all estuaries Marine Stocking in Victoria

Table 3. Estimates of costs associated with additional research required prior to trial releases, including those items listed in Table 2.

¹ NOTE: This cost could be substantially reduced by partnering with other institutions such as state government departments within the species range or the Australian Research Council (ARC).

² Broodstock collection would represent an additional charge to this, depending on the numbers required for spawning.

³Costs have been estimated on the basis of time taken to address these items previously, the associated costs, and calculated at current commercial rates. Please note that these are estimates only and in no way constitute a quote. A specific itemised quote should be sought from service providers when the work is to be undertaken.

Further development of a marine stocking program in Victoria

For each proposed type of marine stocking activity in Victoria a comprehensive Environmental Impact and Risk Assessment needs to be undertaken to clearly and systematically identify and rank risks associated with production of cultured animals; translocation and release of cultured animals; survival of released animals and the environmental/ecological impacts of releases in receiving waters, and returns to recreational fishers compared with the costs of stocking (i.e. a cost/benefit analysis). Such risk assessments will identify the important objectives that any subsequent trial stockings will need to focus on.

A proposed process for proceeding with further development of a marine stocking program in Victoria

At a recent meeting of senior Fisheries Victoria staff¹³, a framework within which the development of marine stocking in Victoria should be managed was proposed. The key elements of the process are:

1. Conduct a comprehensive Environmental Impact and Risk Assessment, including a cost/benefit analysis, for each estuary – species combination. This step would include identifying and addressing all practical investigations that need to be completed (such as those listed in Table 3) before an application to run a trial stocking event can be submitted to the Translocation Evaluation Panel (TEP);
2. Conduct a Translocation Risk Assessment as specified in the Guidelines for Translocating live aquatic organisms in Victoria for each approved estuary – species combination, and develop an application for approval to run a trial stocking event;
3. Once the application to run the trial has been approved, a small-scale trial stocking should be performed accompanied by monitoring of both estuarine fauna and fishery catch. This study should be structured as an additional Environmental Impact and Risk Assessment including a cost/benefit analysis.

A specific list of requirements relevant to Victorian legislation and fishery management needs should be compiled, from which costs associated with this process can be accurately assessed. The development of such requirements could benefit from consultation with recreational fishery managers experienced in marine stocking matters within Industry and Investment NSW.

¹³ Meeting of the Fisheries Victoria Marine Stocking Advisory Group, 3rd Feb 2010, Melbourne. Contact Dallas D'Silva at Fisheries Victoria for additional details.

Conclusions

Based on the information presented here, there are several opportunities in Victoria's marine and estuarine waters where releases of hatchery reared fish could be justified and could produce positive outcomes for the recreational fishery. Fish releases however, are only one tool in the management toolbox and should be compared against, or complement other measures and approaches being used to manage the fishery. These may include provisions to maximise the recruitment of wild stock, such as protection of the spawning stock through sufficient minimum length restrictions or slot limits, closed seasons or spatial closures to protect spawning aggregations, or abolition of fishing methods that lead to mortality of juvenile stock (e.g. trawling in juvenile habitat). Other measures at the catchment scale include sufficient environmental flows, conservation/protection of important fish habitat, and removal of barriers to fish movement. Some of these measures may even negate the need for releases of cultured fish.

At present, fish releases are not included in the various management plans currently in place for Victorian marine and estuarine fisheries, as this novel approach to fisheries management has only recently been proposed to address specific issues. As recruitment limitation is clearly a possibility in some of Victoria's estuaries, fish releases could contribute to the improvement of fisheries in Victoria and may have a role in managing specific instances where recruitment limitation is likely occurring. The role of fish releases should thus be outlined in future revisions of relevant fishery management plans, to ensure they are addressing specific objectives and can be evaluated against these objectives in a quantitative fashion. All recommendations made in this report need to be considered in conjunction with available stock assessments, and where additional surveys/stock assessments need to be conducted.

The next step is to proceed to more comprehensive Risk Assessments for one or more of the opportunities recommended in this study -including environmental impact assessments to meet the requirements of Victorian and national policies for translocation of live aquatic organisms, and cost benefit analyses to determine the economic and social net benefit of stocking. Some areas of research required to comprehensively assess the risks of marine stocking to Victoria's ecosystem are proposed here, but this list is not exhaustive and should be considered in the context of the current translocation guidelines (Anon., 2009b). The Environmental Risk Assessment will likely identify other aspects of species/ecosystem biology/ecology that require further scientific investigation to fully understand these risks. Many of the risks specified in these guidelines would be addressed using the Generalised Predatory Impact Model approach, however the lack of knowledge on stock structure for some of the species considered in this report will require a robust investigation of the genetic structure of the stock, during or after the Environmental Impact and Risk Assessment process. If the program proceeds past the Risk Assessment stage, then development and implementation of any trial releases should be targeted towards optimising release strategies, reducing any identified high risks, monitoring and evaluating the success, costs and benefits of stocking, and understanding the enhancement system. Such an approach is essential to ensure that any larger scale or ongoing releases to follow have a high probability of success and minimal chance of adverse affects. Empirical measurements and assessments of actual releases do not just provide information relevant to the release species. Carefully designed manipulations of species density also provide the opportunity to better understand the dynamics of the target ecosystem (e.g. Fairfax et al., In review), and also to test ecological theories on which the management of our fisheries are based (Walters and Martell, 2004). These additional benefits are often not obvious, but do represent an important consideration of the benefits associated with a release trial.

Acknowledgements

The author would like to offer sincere thanks to fisheries officers who assisted the tour of the estuaries and made themselves available for interview. The remarkable anecdotal knowledge offered by these individuals greatly enhanced my understanding of the systems on which I reported here. I would in particular like to thank that Senior Fisheries Management Officers that assisted, Dick Brumley, and Peter Lawson; as well as Ian Westhorpe. Senior Vic Fisheries staff provided valuable comments on the draft report, and thanks in particular to Murray MacDonald for providing substantial comments on both biological and legislative aspects of the scoping study. Finally, I would like to thank Doug MacFarlane for escorting me along the Victorian coast, facilitating meetings with Fisheries Officers, and assisting with observations and measurements.

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