At the end of the Murray River in South Australia are two large freshwater lakes surrounded by biodiverse wetlands. As a part of Ngarrindjeri Country, Lake Albert and Lake Alexandrina are culturally and ecologically unique, in part due to their close proximity to the Murray estuary in the Coorong. The region is recognised as a Wetland of International Significance under the Ramsar Convention on Wetlands as it is home to a diverse range of habitats and species. Various small islands in the south-west of Lake Alexandrina also help make this special place an ideal landscape for nature. The fringing wetlands of Lake Alexandrina and its islands are home to a vast array of unique plants and animals including the vulnerable southern bell frog, a variety of rare aquatic plants, a diverse zooplankton community (tiny fish food!), and some threatened small-bodied fishes. The habitats supporting this amazing wildlife are highly dependent on careful management of water levels controlled by the operation of gates at five barrage structures, which also stop sea water from entering the lakes.
Southern pygmy perch is a small-bodied threatened fish that can grow to 8 cm long and lives for up to five years in the wetlands of Lake Alexandrina and its islands. The species has a scattered distribution in the Murray–Darling Basin including populations in South Australia, New South Wales and Victoria. The population in Lake Alexandrina is genetically unique from the others so this special population and its habitats warrant well-informed management. Tragically, this population was extirpated from Lake Alexandrina by 2008 during extreme drought conditions. Luckily, just before their habitat disappeared, 65 Southern pygmy perch were collected to become part of a captive breeding program. Thanks to the effort of various people from the South Australian Government, Aquasave–Nature Glenelg Trust and Flinders University, thousands of Southern pygmy perch were reintroduced to the area in 2011–12 which led to the population’s re-establishment in wetlands associated with Lake Alexandrina and its islands.
The status of Southern pygmy perch in Lake Alexandrina has been assessed since 2007 under The Living Murray initiative of the Murray–Darling Basin Authority. The condition monitoring program is managed by South Australia’s Department for Environment and Water, with surveys and reporting conducted by The University of Adelaide. Importantly, these surveys often involve working together on Country with the Ngarrindjeri Aboriginal Corporation.
An aim of the condition monitoring program is to maintain healthy and resilient threatened fish populations, yet monitoring showed Southern pygmy perch abundance was very low in recent years. To understand why there are higher numbers of Southern pygmy perch in some years compared to others we modelled 10 years of survey data in relation to habitat conditions. The model indicated that if the water level of Lake Alexandrina was maintained above a certain level over summer-autumn there would be more Southern pygmy perch captured in our March surveys.
In summer-autumn 2021, the water level of Lake Alexandrina was managed considerably higher than the previous two years when low numbers of Southern pygmy perch were recorded. The March 2021 monitoring results proved fruitful, with four times the number of Southern pygmy perch captured in the surveys than in the previous two years.
These results showed that by maintaining adequate water levels over this critical period when young fish are still growing (following hatching in September), there may be greater recruitment success because more favourable wetland conditions are maintained. If lake water levels fall too low during spring and early autumn, wetland habitats become shallow and water recedes from fringing water plants, meaning the young fish are faced with additional pressures. For instance, the introduced Redfin perch appears to favour the lake environment more than most other parts of the river system. Redfin perch are very good predators of small fish, yet it appears Southern pygmy perch can recruit well in their presence when water levels are adequate because they have healthy water quality, ample food (zooplankton) for fast growth, and abundant aquatic plants they can use to hide.
It is important to note that Southern pygmy perch is only one of many significant species living in the wetlands of Lake Alexandrina and Lake Albert. By measuring the outcome of different water level management, this little fish provides a good indication of the overall health of the wetland ecosystems. Based on our assessment, it is likely that many other species of significance to conservation and Ngarrindjeri culture benefit from water level management in the same manner as Southern pygmy perch, and this will be the topic of future investigations.
Main photo: Southern pygmy perch captured on Mundoo Island.
Photo credit: Scotte Wedderburn.