Habitat, flows and stocking – The triple treat for native fish in Gunbower Creek
Louissa Rogers North Central Catchment Management Authority
Anecdotes from the old newspaper archives dated in the mid to late 1800s, describe rivers and waterholes that were teeming with fish. Large (100 kg+) Murray cod were common and the elders in our communities talk about when they were children asking mum what fish she wanted for dinner and going to the nearest creek or river to get it.
We know that the Gunbower Creek and lower-Loddon River systems once supported 22 native fish species, however monitoring since the mid-2000s shows that only 13 of these species remain. Of these six are listed under state and federal legislation as threatened, including the Murray cod, silver perch and freshwater catfish, or "good eating fish".The waterways within the Gunbower Creek and lower-Loddon River systems were once an interconnected complex of fast and slow flowing waterbodies. However, over the last 120+ years these streams (and many others across the Murray-Darling Basin) have been heavily modified to support the development of irrigated agriculture, rural towns and urban centres.
Dams were constructed to harvest water and store water when it rains over winter and spring, and release water in the warmer months when water is needed for pastures, orchards and crops. Weirs were constructed to divert water via gravity out onto the floodplain or to be pumped directly onto adjacent land.
Some waterways have been dredged to increase the volumes of water that can be delivered through them, in other streams land clearing and stock access cause erosion, which under constant summer flows have silted up deep pools. In the 1970s and 80s rivers were de-snagged to try to improve flood management.
All of these things have made it very tough to for our native fish to survive.
We cannot change things back to what they once were, and why would we want to? Our irrigated agricultural industry is critical to the national economy, and puts food on our tables. So what can we do to support our native fish to survive and thrive, in our working rivers?
Well we know that native fish primarily need three things to support native fish survival.
Habitat – Habitat includes snags, native water plants, deep pools, shallow backwaters, riffles, and also includes well vegetation riparian zones. Habitat is critical for the survival of native fish and provides food, shelter and resting places, substrate for eggs, warm temperature zones for larvae to survive, feed and grow. It also provides refuge areas for all fish during dry periods of low to no flow.
Connectivity: - Fish need to be able to move throughout waterways and associated waterbodies (such as floodplain wetlands and billabongs) to respond to changes in conditions, access food and to complete life cycle processes, such as spawning. The construction of weirs and dams throughout the Murray-Darling Basin has created barriers throughout the system that obstruct fish migration.
Flow – It's a no brainer that fish require flows, but they need flow at the right time, the right rate and the right place. Larvae and juvenile fish require slow stable flowing warm shallow water to access food and provide protection from predation. Flows in winter prevent juvenile fish that have spawned or were stocked from becoming food for adult fish in the winter pools by ensuring access to edge habitat that provides food and protection. Rises in water levels in spring cue fish to migrate for spawning, high flows in late summer and autumn encourage juvenile fish to disperse, ensuring genetic viability throughout the system.
The North Central CMA has a plan of action for the Gunbower Creek and lower-Loddon system; a Native Fish Recovery Plan. The plan identifies key actions that will ensure that the three cornerstones of native fish survival are addressed. Together with our partners, and along with complementary activities such as stocking, we are constructing fish passage on weirs, reinstating snags, fencing and revegetating the river banks, exploring the recreation of deep pools, and managing flows.
Sounds simple enough? Except that the Gunbower Creek and Pyramid Creek (a major tributary to the lower-Loddon River) are major irrigation water carriers in the Torrumbarry Irrigation Area. These creeks became part of the irrigation system in 1923, with the construction of the Torrumbarry Weir. Since then they have been operated just like channels for the supply of water for irrigation and towns. In Victoria, a Statewide method (referred to as the FLOWS method) is used to establish ecological objectives and determine the environmental water requirements using the natural flows that would have occurred dams were not in place as a basis for setting these flows. However, under the typical flows delivered for irrigation, water is delivered when there is irrigation demand from late spring to early autumn, and the creeks cease to flow during winter when the irrigation system is shut down. The challenge, in such highly regulated systems, is to deliver environmental flows without impacting on irrigation supply, which means we can't reinstate the natural flow regime.
In Gunbower Creek, the North Central CMA has used the scientific understanding of the Murray cod life history to plan and deliver flows. We have targeted Murray cod, because our monitoring has shown a serious decline in Murray cod numbers, and most of the fish being recorded in our surveys were older, which indicates that young fish that either naturally spawned or were stocked in the previous spring were not surviving.
The life cycle of the Murray cod has the following vitally important flows - winter flows, spring rise, stable flows, engaging the littoral zone and ramp down.
Of these, the winter flow component was the easiest to achieve as we are the only player in the system during the off-irrigation season, so can achieve this target without impacting on other water users. Irrigation demand and supply generally achieves the spring rise, littoral zone and ramp down flow components.
The stable flow component is the most challenging. Review of the flow data at Cohuna Weir showed that irrigation demand creates very rapid and extreme fluctuations such as a change in flow by over 600ML/d within a two hour period. To combat this, we set our target environmental flow rate quite high, well above the usual irrigation demand, so our flow operators had the flexibility needed to be able to achieve the stable flows. This enabled us to smooth the hydrograph and reduce sub-daily variation. While this is not what traditional flows studies have recommended - FLOWS studies often suggest that during summer flows in streams should be very low as would have occurred naturally to provide shallow, relatively still warm water for larvae and juvenile fish to feed and grow – over the last four years since delivering flows to support the Murray cod life cycle, we have seen young fish, of each year age class, being recorded each year in our surveys. This is good news for the fish, and good news for the anglers.
In Pyramid Creek a similar approach was used to provide flows to stimulate fish movement, particularly golden and silver perch, through the lower Loddon River and the Pyramid Creek system.
Pyramid Creek was dredged in the 1960s to create a greater channel capacity, destroying the creek bed variability by turning the creek into a canal. Science has told us that Pyramid Creek is an important pathway between the Loddon River system and the Gunbower Creek system, with the high value wetland habitat Kow Swamp, an important environmental asset connecting these two systems, which is potentially great nursery habitat for native fish such as perch. Under the Native Fish Recovery Plan, using a mix of recreational fishing grants and Riparian Action Plan funding, we have reinstated around 36 snag complexes into Pyramid Creek to provide resting points for fish moving almost 70 km through the creek between the Loddon River and Kow Swamp. Flows in Pyramid Creek meet the Loddon River at Kerang Weir, where water for irrigation is diverted to the Kerang Wetlands, west of the Loddon River. Other than a passing flow over Kerang Weir, the only source of water from Kerang to the Murray River is water for the environment.
Our flow planning for Pyramid Creek focusses on three flow components. As with Gunbower Creek we will reinstate the winter base flow, this will ensure that the snag complexes remain engaged and any fish in the system are not stranded. The other flows we have targeted are two high flow events, one in autumn and one in spring, to stimulate fish movement through the fishways on Kerang Weir and Box Creek regulator into Kow Swamp.
We have delivered two of these high flows to date. To ensure we do not compromise the needs of irrigators we delivered environmental water on top of the irrigation water requirement to Kerang Weir. To supplement the water that was diverted for irrigation to the Kerang Lakes, water was also delivered to meet the flow at Kerang Weir, from the Loddon River. Combining these flows provided the balance between consumptive water in Pyramid Creek, and the flow required downstream of Kerang Weir to encourage fish movement.
The resulting flow rate at Kerang Weir stimulated the movement of large and small bodied native fish species through the fishway at Kerang Weir – some moved upstream into the Loddon River, and others through Pyramid Creek and into Kow Swamp. We need a few more years of data to understand the benefits that this movement is having to the broader fish population, but we have found that some very large Murray cod have decided that our new snags are a great place to live.
The evidence that we are collecting from monitoring of these flows shows that we can have it all; healthy productive irrigation communities, and a world class Murray cod trophy fishery.