Key words: Environmental flows, floodplain wetlands, fish migration, regulators, Native Fish Strategy.
Passage to and from floodplain wetland areas is very important for native fish, as some species utilise these areas for spawning, feeding and recruitment (survival of fishes from eggs to reproductive stage). Regulators were originally installed in the Barmah-Millewa Forest (BMF) to keep water out during the agricultural irrigation season (Fig 1), however such regulators are now thought to adversely effect the lateral movements of native fish.
Broad aim and methods. This project aimed to investigate the lateral movements of native fish during normal river discharges and during an environmental water allocation (EWA) in order to determine the impact of regulators on native fish movements in the BMF.
Sampling was conducted within a number of key fish species in the BMF. Electrofishing was used to determine the presence and abundance of species within habitats sampled (Fig 2). Egg samples were taken from adult individuals to understand whether fish in regulated offstream habitats developed and spawned naturally. Tagging (radio-telemetry and t-bar tagging) was used to monitor movement behaviour, and drum, fyke, frog and larval nets were used to determine which species are using the waterways sampled and to investigate whether movement is influenced by flow. Water quality parameters were also recorded.
Findings: Results suggest that movement between the Murray River and wetland creeks occurs on a regular basis in unregulated parts of the BMF. Fish generally respond to changes in flow by moving into these creeks on rising flows and returning on falling flows.
In regulated systems, fish generally approached the regulator on falling flows, presumably trying to return to the Murray River like fish in unregulated creeks, but being prevented from returning by the regulators they therefore remained stranded downstream. Movements back to the Murray River were only possible for a few large-bodied individuals during flood conditions when regulators were drowned-out and water velocities and turbulence reduced to the extent that such fish could escape.
Rising water levels associated with the EWA induced fish to move into unregulated wetland creeks and fish generally occupied these creeks for the duration of the EWA. Results indicated that fish remain in these creeks for as long as they are inundated to spawn and feed. Modelled data suggest that without the EWA, flows would have dipped below bankfull several times in late 2005, which, based on previous movements, would most likely have resulted in fish leaving wetland creeks, possibly interrupting feeding and/or spawning activities.
Lessons learned and future directions: Results from this project will be used to guide the use of off-channel regulators to facilitate movement of native fish to and from off-channel habitats to promote spawning and reduce the likelihood of fish being stranded in drying off-river habitats. The results of this project will also provide information for better targeting of environmental water releases for native fish.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy.
Contacts: Dr Matthew Jones, Arthur Rylah Institute for Environmental Research (ARI). (03) 9450 8600, email@example.com