Nationally threatened Yarra Pygmy Perch confirmed in the Upper Wannon River

The Yarra Pygmy Perch (Nannoperca obscura) is a small freshwater fish (the biggest one I have seen was 78 mm in length). It is endemic to coastal waterways of south-eastern Australia and prefers slow flowing habitat along the edge of streams and rivers, and also permanently inundated wetlands – but they don’t tend to move around much, neither as adults or larvae.

The species was probably first recorded in or near the Yarra River, hence the reference in their name, but was actually much more widespread: ranging from central Victoria and all the way across to the Murray River in South Australia. The species has declined since European settlement and it is is currently listed as vulnerable (IUCN and EPBC Act 1994), protected and critically endangered in South Australia, vulnerable in Victoria and has a National Species Recovery Plan. While considered to be more widespread historically, habitat fragmentation has been exacerbated by recent drought and is considered a major threat along with competition and predation by introduced species and climate change.

Despite its overall decline, at NGT we have worked closely with this fish in a number of regions (see Murray Darling Basin example here). In the far south-west coastal zone of Victoria, this has included some positive outcomes for fish at Long Swamp, one of NGT’s restoration sites near the SA-VIC border. To learn more about the wider wetland restoration project at Long Swamp, you can read the full story here. One of the champions of this restoration story is the Yarra Pygmy Perch. It is my opinion that Long Swamp now contains the highest localised abundance of this species across its entire range, and we have witnessed their expansion and recovery throughout this system first hand.

Yarra Pygmy Perch in breeding condition at Bully Lake (Long Swamp)

In a drier period at the start of 2016 we undertook some surveys of the Upper Wannon River as part of two projects funded under the Victorian State Governments Threatened Species Project Initiative. Owing to the dry conditions I was particularly interested in anywhere that still held water and I came across a couple of small fire dams adjacent to the Wannon River, within the Grampians National Park. To my surprise these small dams contained Southern Pygmy Perch, Little Galaxias and what looked like Yarra Pygmy Perch. In all of our sampling of the Upper Wannon and associated wetland complex (Brady Swamp, Gooseneck Swamp and Walker Swamp) we hadn’t come across Yarra Pygmy Perch, so this potential record seemed to be a real outlier.

A small fire dam next to the Wannon River acting as drought refuge for native fish

Over the past two years we have undertaken further surveys to increase our understanding of the ecohydrology of the Upper Wannon River (funded through the Australian Government’s Australian Heritage Grants Program 2020-21) and it has offered a chance to revisit the occurrence and status of this outlying population of the species. While the initial surveys picked up only a handful of fish, surveys in 2021 and 2022 found a lot more. However they do seem to be concentrated around the same limited geographic area. Just to be sure, I collected some tissue samples and have now had their identity confirmed through DNA sequencing (thanks to enviroDNA).

Yarra Pygmy Perch (top) and Southern Pygmy Perch (bottom) from the Upper Wannon River. Note the smaller mouth, more oval eye and lighter, gold colour of the Yarra Pygmy Perch.

The confirmed presence of this localised population does raise questions as to why we aren’t seeing them further downstream, particularly in Brady Swamp but this is probably further confirmation that the species has quite a low capacity to move. It will be interesting to see what we catch later this year when we undertake a more comprehensive fish survey of these wetlands, following on from the highest sustained flows recorded in the Wannon River in recent memory.

Aside from how our understanding of their distribution in this watercourse unfolds, the confirmed presence of this species provides a very strong justification for having a more sophisticated discussion about a more nuanced approach to flow management in the future, including the operating rules that currently divert flows away from the headwaters of the Wannon River each winter and spring. The fact that this species was only just holding on in 2015/early 2016 means that any capacity to maximise flows in drier years is critical, in order to hydrate and replenish wetlands and waterholes that are utilised by the species as key drought refuge habitat.

Lachlan Farrington