River regulation has reduced the frequency of inundation for many wetlands across the Murray-Darling Basin, contributing to a decline in biodiversity and native fish populations. Ephemeral wetlands – or those wetlands which have natural wetting and drying phases – offer critical nursery habitat for larval native fish. Environmental water can be delivered via pumps and regulators to these wetlands to create ideal conditions for native fish to survive and grow. These benefits are enhanced when environmental water is used with other management actions including the exclusion of pest fish species, the use of native fish stocking programs, and when complementing natural wetting and drying cycles.

This article explores the importance of wetlands for native fish and provides examples of how multiple management actions are giving our young native fish a fighting chance.

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Key points for managers:

• Natural resource managers deliver environmental water to Murray-Darling Basin wetlands to support native flora and fauna.
• The periodic inundation of ephemeral wetlands intermittently creates habitat diversity with heightened survival, feeding and reproduction opportunities for waterbirds, frogs, turtles and water bugs.
• Wetlands that receive environmental water also benefit native fish by providing productive nursery habitat in which growth from tiny larvae to adolescent or adult fish is supported (which is called ‘recruitment’).
• If pest carp are excluded during filling of a wetland (using screens on inlets or pumps) there can be stark improvements in water quality leading to stronger growth of aquatic plant and plankton communities. This supports recruitment of native fish, which can enter the wetland through screens or pumps, by providing structure to hide in and abundant food sources.
• Environmentally watered wetlands in which adult carp invasion has been limited can support recruitment of species like golden perch, silver perch, freshwater catfish, and Murray cod – or even threatened species such as Murray hardyhead or southern pygmy perch.
• Managers can consider stocking hatchery bred native fish into these wetlands where natural colonisation does not occur.

Wetlands and fish

Floodplain wetlands offer productive breeding and feeding habitat for a variety of organisms, supported by the seasonal (or ephemeral) drying and wetting cycles which fuel productivity. The drying and wetting of ephemeral wetlands is linked to the seasonal variations in river flows, particularly in slower flowing ‘lowland’ stretches of rivers in the Murray-Darling Basin. When high flows or floods inundate dry wetland sediments, a pulse in nutrients is released - a bit like a million stock cubes being wetted. This pulse triggers a ‘boom’ in plankton and aquatic plants which had lay dormant in the dry sediment or arrive as propagules in the inflows. Plankton are tiny organisms that many critters – including baby fish - depend on as food in the early weeks of their development (Figure 1). Aquatic plants provide places for small animals to hide from predators, and produce dissolved oxygen in the water via photosynthesis. This dissolved oxygen (or ‘DO’) is critical for many aquatic critters which ‘breathe’ the dissolved oxygen using gills. In short, the periodic draw down and re-inundation of ephemeral wetlands creates diverse and productive habitat which supports heightened survival, feeding and recruitment opportunities for native fish.

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Unfortunately, river regulation has reduced the frequency of inundation for many Murray-Darling wetlands, contributing to a decline in biodiversity and native fish populations across the Murray-Darling Basin. To make things worse, non-native carp are early colonisers of newly inundated wetlands, disrupting plant germination and plankton emergence, and increasing turbidity (murkiness), which in turn impacts aquatic plant photosynthesis. These factors significantly reduce opportunities for and success of wetland recruitment by native fish.

Managing Wetlands for native fish

Wetland rehabilitation programs often involve managing wetting and drying cycles (the hydrological regime), particularly where a wetland supports key species, ecological communities or cultural and/or social values. The importance of ephemeral wetlands in supporting the growth and survival (‘recruitment’) of native fish has been explored in recent decades through a series of pilot projects or case studies (some involving the author). Research investigating recruitment and dispersal processes of species like golden perch, Murray cod, and silver perch continues to highlight the value of wetland habitats to native fish recruitment (see further reading list).

Typically, natural resource managers use flow control structures (regulators) to deliver water to a wetland,or to isolate it to initiate a dry phase. Pumps are also used to deliver water to logistically challenging wetlands in sections of rivers where regulated river flows rarely reach anymore. Preventing pest carp (and pest redfin and gambusia) from entering wetlands as they are filled can lead to better water quality, plankton emergence, and aquatic plant establishment. Some wetland regulators incorporate screens which prevent larger carp from entering as the wetland fills. Read more about fish screens here.

Pumping water to wetlands also limits invasion by adolescent and adult carp, while strategic placement of pump suction points (e.g. deeper water, outside river bends) can also reduce the likelihood of transferring baby carp or Mosquito fish to the wetland. A case study in Tar-Ru Wetland on the Lower Murray River in 2016 trialled temporary screening using wire mesh. The mesh was installed by local First Nations rangers in collaboration with NSW DPI Fisheries during a wetland dry phase.When the wetland subsequently filled, adult carp were successfully excluded,resulting in improved water quality and development of zooplankton communities(Figure 4). Around 700 hatchery-bred golden perch fingerlings were later stocked into the nursery wetland, demonstrating rapid growth upon recapture several months later, before flooding provided the opportunity for them to disperse as adolescents to the river itself (see Ellis et al. 2017).

Furthermore, by timing filling events to coincide with higher river flow events in spring-summer, the potential for transferring drifting eggs and larvae of fish like golden perch and silver perch into the managed wetland is increased. This was documented in a 2009 case study, when surveys identified juvenile golden perch and silver perch in a Murray River wetland that had been pumped full after a dry phase(see Ellis and Pyke 2011) (Figure 5). After the pump deposited them in the wetland as eggs or larvae, these young demonstrated rapid growth in the warm plankton rich waters over several months. Efforts to trigger their movement back to the river by opening the wetland regulator and allowing a partial drain of the system resulted in relatively few fish exiting the wetland (mind you,plenty of carp were documented moving from the river into the wetland,attracted by the nutrient rich outflows).

Interestingly, when the regulator was shut and the wetland was topped up again via pumping, a strong movement back towards the river in the fresh inflows by adolescent golden perch and silver perch was detected. This makes some sense ecologically for fish species that historically spawned on rising rivers, with young often drifting into wetlands during flooding. After a period of rapid growth in the warm and food rich nursery, they would exit as adolescents next time the river reconnected with their nursery, joining the larger adult cohorts in flowing streams while the next generation of larvae replaced them back in the wetland. It is not a perfect strategy – a nursery wetland may dry out in a drought stranding young fish – but that’s how nature works. Given the strength of cohorts of fish that can develop in the Murray-Darling Basin following flood years, the strategy clearly works over the longer term.

In recent decades, managers and Fisheries agencies have started to consider stocking of hatchery bred native fish such as golden perch fingerlings or threatened species into recently inundated wetlands where young are unable to colonise naturally during the filling event. Environmental programs using carp exclusion screens or pumped water delivery often create pest free habitats in which this option may be suitable. After a period of growth in the wetland, developing fish return to the river during natural flow events that re-connect the wetland or managed top-up or draining events via wetland regulators (see Figure 6). Opportunities to surcharge a wetland by pumping to ‘fill and spill’ are also being explored.

Stay tuned for more information about a Victorian Fisheries Authority project exploring these opportunities in 2025.

Further reading

Ellis, I. (2017). Managingwetlands for native fish - Lock 8 Wetland 780 Fish Nursery Project on Tar-RuLand. Completed for the Western LLS by NSW Department of Primary Industries -Fisheries, Buronga.

Ellis I and Pyke L (2011). Assessmentof the fish community in Margooya Lagoon post Murray River flooding and lateralmovement upon reconnection to a rising Murray River. Report prepared for theMallee CMA by The Murray-Darling Freshwater Research Centre.

Ellis I, Huntley S, LampardB (2014). Fish movement in response to hydrological management of ButlersCreek, Kings Billabong Nature Reserve VIC. Report prepared for theMurray–Darling Basin Authority by The Murray–Darling Freshwater ResearchCentre, MDFRC Publication 34/2014, July, 38 pp.

Ellis, I., Cheshire, K.,Townsend, A., Danaher, K. (2022). NSW DPI Industries. Fish and Flows in theSouthern Murray-Darling Basin: Developing environmental water requirements fornative fish outcomes.

Stuart, I.G. and Sharpe,C.P. (2020). Riverine spawning, long distance larval drift, and floodplainrecruitment of a pelagophilic fish: A case study of golden perch (Macquariaambigua) in the arid Darling River, Australia. Aquatic Conservation: Marine andFreshwater Ecosystems 30(4), 675-690.

Baldwin, D. and Mitchell,A. (2000). The effects of drying and re-flooding on the sediment and soilnutrient dynamics of lowland river–floodplain systems: a synthesis. RegulatedRivers: Research and Management 16.

Hillyard, K., Smith, B., Conallin,A. and Gillanders, B. Optimising exclusion screens to control exotic carp inan Australian lowland river. Marineand Freshwater Research, 61, 418–429

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The information contained in this article is based on knowledge and understanding at the time of writing (January 2025). However, because of advances in knowledge, readers are reminded of the need to ensure that the information upon which they rely is up to date, and to check the currency of the information with the appropriate staff at Millewa Pumping Company or the user’s independent adviser.