Anglers tune in to find out why Port Phillip Bay is so important to snapper

Executive summary

The Port Phillip Bay snapper fishery is highly valued by anglers, and is a major drawcard for recreational fishing in Victoria. The adult fishery is highly seasonal depending on migration of fish to and from the bay and coastal waters, and their aggregation in certain regions of the bay for spawning in spring-summer. The bay is also a juvenile nursery area, and anglers target smaller fish both during and outside of the main adult fishing season. The seasonal and spatial dynamics of this fishery are clearly linked to fish movement patterns and habitat use. Developing a more detailed understanding of movement behaviour, and what might influence it, is important because changes to movement behaviours will impact on the recreational snapper fishery.

Acoustic telemetry was applied to study the movement behaviour of adult and juvenile snapper at multiple scales of space and time. Acoustic telemetry involves surgically implanting acoustic transmitters ('acoustic tags') into fish and detecting their locations over time with arrays of strategically located monitors ('listening stations') moored on the seabed. The listening stations detect the presence of the transmitters within a certain detection radius (typically 300-400 m) and log data on the dates, times, and unique tag identification numbers for each fish. Over approximately 3 years, information derived from 85 adult and 57 juvenile tagged snapper, with over 700,000 detections across a core group of 47 listening stations, was analysed and interpreted to provide information on:

1. Migratory dynamics of snapper between Port Phillip Bay and coastal waters,
2. Movement of snapper within Port Phillip Bay, with emphasis on patterns of residency in the main spawning and fishing region,
3. Relationships between movement patterns and sea surface temperature (SST), and
4. Habitat use, focussing on comparing patterns of use of natural and artificial reefs, and sediment habitats.

Other insights into migration behaviour and habitat/area use were obtained from detailed consideration of individual fish
movement histories.

Adult snapper demonstrated highly evolved and systematic annual migration behaviour between the bay and coastal waters that can be reliably predicted based on SST patterns:

• Arrival of adults into the bay began in September and peaked in October, occurring over an SST range from 13-16°C.
• The main period of adult departures was mid-December to February when SST exceeded 19°C, and a second period of departures occurred in April-May as the SST decreased.
• Consistent association of adults within the Carrum Bight–Hobsons Bay region during the peak spawning period (November-December) confirms the importance of this area as a major spawning ground.
• Repeat migrations of individual adults to and from the bay across multiple spawning seasons were demonstrated unequivocally for the first time.
• Repeat migrants used similar areas in different years, and showed remarkable similarity in timings of arrival and departure.
• Two types of residency patterns for spring-summer immigrants were detected: short-stayers (the majority of fish) that stay until December-January, and long-stayers that stay until April-May.
• Evidence of some overwinter adult residency was found, but this was likely underestimated, and
• Re-aggregation of adults in deeper waters off Mornington occurred in late summer/autumn and appears related to a secondary spawning period as the water temperature declines.

The high frequency of use of artificial reefs indicated that these structural features are important for adult and juvenile snapper. However, the wide and sporadic movement by adults suggests that spawning likely occurs opportunistically throughout the north and east of bay, rather than at specific locations. Natural reef was utilised all year by juvenile snapper, and there was a general pattern of movement of juveniles from artificial to natural reef in the north of the bay in autumn. Greater use of this habitat by juveniles over the autumn/winter months may relate to it providing better feeding opportunities, and or more cryptic refuges in the colder months when their activity reduced. Artificial reefs appear to provide important transitional habitat for smaller juveniles and juvenile aggregation areas over the spring-summer.

Adult migrations appear cued by seasonal changes in temperature/photoperiod so that spawning occurs when egg/larval stages can develop in optimal temperature conditions of 18-22°C. Changes in climate will have implications for these behavioural patterns. Information from this study will be important for predicting impacts of sustained warming or changes in annual water temperature cycles on the Bay's snapper fishery and spawning success. The refined understanding of migratory dynamics and spatial habitat use within the Bay provides context for assessing efficacy of spatial and temporal management approaches. Finally, anglers can use monitoring of SST, and a refined knowledge of movement dynamics, to inform how they effectively allocate their fishing effort over the annual cycle.

Further information

For a copy of the full report please email richard.rogala@vfa.vic.gov.au