Mussels in the Avon – What Might Be Living in Our River?
Most people don’t give freshwater mussels a second thought. They’re small, unobtrusive, and easy to overlook – but they are among the most effective water purifiers in any river system. A single 100-gram animal can filter more than 12,000 litres of water a year, removing algae, bacteria, and suspended particles, and transferring nutrients to the sediment where they can be processed naturally. Multiply that across a healthy population and you have a living filtration system of remarkable scale.
The Beverley River Care Group has been learning about what might be possible in the Avon, and the news is more hopeful than you might expect.
What we know is already there
In December 2023, researchers from DWER’s River Science team recorded a Fluviolanatus subtortus – a small native freshwater mussel – at Gwambygine Pool on the Avon, downstream of the confluence with the Dale River. The species has also been found in the Dale River at Reserve Pool. These are significant findings. They tell us that at least one native mussel species is clinging on in our river system, in conditions that suit it.
Fluviolanatus subtortus is a small mussel that attaches to hard substrates – snags, rocks, any firm surface near the sediment. Less is known about this species than the others, but it has been found in some unexpected locations, including well up the Murray River. If you know what to look for, you can find them: run your hand along submerged timber or rock near where it meets the riverbed, paying particular attention to crevices. If mussels are present, the surface will feel rough. Look closely and you’ll see shiny brownish-black shells – small, but distinctive once your eye is in. This same approach works for finding Black Pygmy Mussels too. https://www.inaturalist.org/taxa/1209242-Fluviolanatus-subtortus
Carter’s freshwater mussel – a harder story
The other native species with a long history in wheatbelt rivers is Westralunio carteri – Carter’s freshwater mussel. It has the remarkable distinction of maintaining the lowest internal salt concentration of any known animal. That extraordinary adaptation, which made it perfectly suited to freshwater systems, is also its greatest vulnerability: rising salinity has been the primary driver of its loss from rivers across the wheatbelt, and an animal with so little tolerance for salt has essentially no buffer when conditions deteriorate.
The ongoing secondary salinisation of wheatbelt rivers, a consequence of broad-scale land clearing, continues to push salt levels higher over time. Periodic discharges of hypersaline water from storages such as the Yenyening Lakes add directly to that load, with real consequences for water quality in the reaches around Beverley.
The River Training Scheme of the 1970s compounded these pressures in a different way. For Carter’s mussel, which lives in and on the sediment rather than attaching to hard structures like other mussel species, the loss of snags was less about direct habitat destruction and more about what disappeared with them: shade. Riparian vegetation and submerged timber moderate water temperature and reduce evaporation in the pools that remain when the river stops flowing over summer. Without that canopy, those pools become increasingly hostile – particularly for an animal already pushed to its limits by salt.
Carter’s mussel is now red-listed as a threatened species and has been lost from the Avon, the Blackwood, and the Murray. Some populations survive in freshwater tributary creeks that haven’t been reached by salt, but reestablishing them in the main Avon channel faces serious obstacles – regulatory, ecological, and practical. Contributor to this blog post, Alan Cottingham (Research Fellow, Centre for Sustainable Aquatic Ecosystems, Harry Butler Institute, Murdoch University) also mentions that Westralunio carteri has a fascinating life cycle involving a parasitic larval phase and a fish host – but that, he says, is a story for another day.
Carter’s Mussels on iNaturalist https://www.inaturalist.org/taxa/115023-Westralunio-carteri with photos.
Learning from the Canning
A recent project on the Canning River, involving The Nature Conservancy and Murdoch University’s Centre for Sustainable Aquatic Ecosystems, showed what is possible when conditions are improved. Rather than relocating mussels – which requires stringent regulatory approval and is rarely achievable across different river systems – the project focused on creating suitable habitat: providing the physical structures mussels need to colonise naturally. The results were encouraging with the endemic Black Pygmy Mussel or Brackish Water Mussel (Xenostrobus securis) https://www.inaturalist.org/taxa/493896-Xenostrobus-securis.
That distinction matters. Translocation of aquatic animals in Western Australia is tightly regulated, and for good reason – introducing species from one system to another, even native species, carries ecological risks. The more promising path for the Avon is to find what is already present, understand where it’s surviving and why, and then support those populations by improving habitat and water quality conditions.
Our project: surveying upstream of the Dale confluence
The key question we now want to answer is whether mussels are present upstream of the Dale River confluence – in the stretch of the Avon from Qualandary Crossing down to where the Dale meets the main channel.
The freshening effect of the Dale River appears to create conditions suitable for Fluviolanatus subtortus near and below the confluence. Further upstream towards Beverley, salinity is generally higher, and conditions harder. But pools do persist, conditions vary seasonally, and it’s worth looking.
We’ll be working with guidance from researchers at Murdoch University and DWER’s River Science team as the project develops. If you’ve seen anything in the river that might be a mussel shell – live or dead – we’d like to hear from you. Dead shells near a pool are often a sign that a species was present recently, or may still be.
Email us on hello@beverleyrivercare.org.au
Why it matters
The Avon is under pressure. A drying climate is reducing flows, pools are shrinking, and water quality suffers when there is less movement and less biology to support natural filtration. Managing the inputs of highly saline water into the system – whether from natural catchment processes or from infrastructure discharges – is an essential part of any long-term recovery. Mussels are not a solution on their own, but a healthy mussel population – even a modest one – would be a meaningful indicator that the river is in better condition, and would actively contribute to keeping it that way.
The veldt grass comes out by hand. The trees go in one at a time. This is the same kind of work: patient, local, and worth doing.
Sources included:
Centre for Sustainable Aquatic Ecosystems, Harry Butler Institute, Murdoch University
River Science Team, Department of Water and Environmental Regulation
York River Conservation Society
Friends of the Dale River