Where have all the freshwater mussels gone?

Author: Margrit Beemster

Photo credit: Michael Griffin and Nicole McCasker, Charles Sturt University

It wasn’t that long ago that freshwater mussels numbered in their “hundreds of thousands” throughout the Murray-Darling Basin.

“Most people I’ve spoken to have stories of mussels being everywhere when they were children, but now not so much,” says Charles Sturt University researcher Dr Nicole McCasker who recalls there were plenty about when she was growing up in Kerang, on Perrepa Perrepa or Barapa Barapa and Wamba Wamba or Wemba Wemba country, in southern Murray-Darling Basin.

Nicole is a freshwater ecologist and leads the fish monitoring component of the Commonwealth Environmental Water Holder’s science program Flow-Monitoring Evaluation and Research (Flow-MER) in the Edward/ Kolety-Wakool river system. She is also one of the few researchers in Australia who focus on the ecology and conservation of inland mussels.

“Unfortunately, we have very little understanding of the current distribution and size of freshwater mussel populations,” says Nicole, who became interested in researching mussels six years ago.

Dr Nicole McCasker with a freshwater mussel.
Freshwater mussel out of water.

To help address this gap in our knowledge, Nicole leads a research project trialling the use of side-scan sonar, a technology familiar to many recreational fishers for finding fish, to detect freshwater mussel beds.

In September and October 2023, researchers from Charles Sturt University and Austral Research and Consulting undertook side-scan sonar surveys, mapping five two kilometre reaches of river bottom in Yallakool Creek and the upper Wakool River. A second round of surveys were conducted using different sonar equipment that produces a higher resolution image, to try and confirm if the “bright spots” identified in the images from the first survey were actually mussels.

“We are trialling these methods,” says Nicole. “It’s a bit like looking for a needle in a haystack, only we don’t know how many needles there are or if there are any at all! We know that this technology has been used overseas to identify areas where mussels are likely to be, but this appears to be the first time it has been used in Australia. If this survey method works in inland rivers, it has the potential to be used elsewhere in Australia.”

Dr Nicole McCasker searches for mussels in Yallakool Creek.

Specifically, the researchers were looking for the two widespread species of large, burrowing freshwater mussels in the southern Murray–Darling Basin – the river mussel Alathyria jacksoni that grows to about 20 cm; and the billabong mussel Velesunio ambiguus, that grows to about 12 cm.

“Mussels play an important ecological role in rivers and wetlands,” says Nicole. “They are filter feeders so they improve water quality, provide habitat for other animals, and create resources for other organisms.”

Mussel shell fragments.

Freshwater mussels are also of cultural importance to the First Nations peoples of the Murray–Darling Basin including local Traditional Owners of Edward/Kolety-Wakool area where the study was conducted, the Perrepa Perrepa or Barapa Barapa, and Wamba Wamba or Wemba Wemba peoples.

As Jeanette Crew, OAM, chairperson of the Yarkuwa Indigenous Knowledge Centre at Deniliquin and a Wamba Wamba Pereppa Pereppa Nation woman says: “Mussels were highly significant for our people. Not only were they a major food source, their shells were also used as implements for cutting, scooping and fishing, and for ornamental purposes such as necklaces and that sort of thing.”

There are huge middens with mussel shell fragments in some parts of the Murray including in the Barmah-Millewa Forest and the lower Murray. “We are also finding mussel shells and fragments on the riverbanks during our ongoing river monitoring as part of the Flow-MER program,” says Nicole.

What Nicole also noticed, during the ongoing monitoring of larval fish as part of the Flow-MER program, was the presence of ‘glochidia’, on the larval and juvenile fish they were collecting throughout the region.

Glochidia is the term used to describe the early life stage of a mussel. After being released from the female mussel, the glochidia attach themselves/parasitise to native fish for a few weeks. During this time, they get the nutrients they need to develop before dropping off the fish as a fully formed juvenile mussel.

“Marine mussels do not have this life cycle,” explains Nicole, “but the large freshwater mussel species including both the billabong mussel and river mussel ‘have to have fish’ to complete their life cycles. I have been doing research on freshwater fish for 15 years but hadn’t been looking for glochidia. But once I began thinking about glochidia, I started to see them on larval fish. There they were, in front of me the whole time.”

A glochidium (approximately 0.25 mm in size) attached to the tail of a fish host (in this case, a carp gudgeon).

When Nicole reviewed the Edward/Kolety-Wakool Flow-MER project’s larval fish collection back to 2018/19 she found the glochidia were more common on samples from certain sites.

“So, the next question is, where are the adult freshwater mussels?” says Nicole. “We know they are very important ecologically and culturally but, as far as I know, little if any surveys have documented their presence in the Edward/Kolety Wakool system. One hypothesis is that the adult mussels may be in the same location where we saw large numbers of glochidia. So, as adult mussels are difficult to survey, then maybe the glochidia might be a good indicator.”

The team from Austral Research and Consulting conducting side-scan sonar mapping of Yallakool Creek.
Nicole McCasker records potential mussel sites.

Finding adult mussels in the lakes and rivers of the Murray–Darling Basin isn’t easy.

“In clear, shallow river systems researchers can snorkel and visually count freshwater mussels under the water,” says Nicole. “However, our inland river systems are deep, turbid with lots of snags. Surveys for adult mussels are currently limited to systems that are shallow enough to search for mussels by feeling with your hands or feet. Our research project aims to trial side-scan sonar to see if we can detect adult mussels in area that would otherwise be difficult to sample.

“It’s all about trying to find out where have the mussels gone and how we get them back. Globally freshwater mussels are one of the most threatened species alive, but for different reasons. As freshwater scientists we think river regulation and other changes to rivers in the Murray-Darling Basin has contributed to the decline, so then the next step is to ask ‘what watering/flow regime do we need to help bring mussels back?’ Understanding where they are and where they are not, will help us to inform environmental water managers.”

Using industry technology for science

Searching for freshwater mussels is something new for aquatic ecologist Dr Dion Iervasi and survey technician Paul Davies, from Austral Research and Consulting, a business which usually does fish surveys, habitat mapping, and bathymetry modelling.

Dion co-supervises, together with Dr Nicole McCasker, a CSU Honours student (Jack Nicholas) who is investigating the reliability of using sonar to count mussel numbers.

Paul Davies (left) and Dr Dion Iervasi, from Austral Research and Consulting, show Dr Nicole McCasker, CSU, images form the side-scan sonar.
Aquatic ecologist Dr Dion Iervasi and survey technician Paul Davies, from Austral Research and Consulting, conducting side-scan sonar surveys in the upper Wakool River.

“This mussels project in the EKW system, where we used side sonar scanning, kind of piggybacks on the honours project,” says Dion. “It’s an additional application of some of the technology we have which usually doesn’t get used in ecology but mostly for mapping oceans for navigational surveys.”

As Paul, whose expertise is in sonar mapping says: “We are taking it out of that industry and deploying it into more of a science-based ecology focus.”

“For me, as an aquatic scientist, anything where we don’t know the answer for is interesting. This is an ecological question that we don’t know about, not sure we understand…. but if what we are doing can help with the understanding and then management of these systems then that’s a good outcome.” – Dr Dion Iervasi

Sharing knowledge

A Knowledge Sharing Day on freshwater mussels was held at the Yarkuwa Indigenous Knowledge Centre in Deniliquin on October 17, 2023.

The day was an opportunity to share knowledge with local Traditional Owners – the Perrepa Perrepa or Barapa Barapa and Wamba Wamba or Wemba Wemba peoples. Dr Nicole McCasker, Yarkuwa staff and Kolety-Werkul rangers talked about the types of freshwater mussels in the region, their ecology and important roles they play in their ecosystems.

Kolety Werkul River Rangers Brandon Cooper (left) and Shemar Day (right), with Dr Nicole McCasker (CSU) check out the images from the side-scan sonar.
Kolety Werkul River Ranger Brandon Cooper looks for glochidia under the microscope.

Jeanette Crew, Crew, OAM, chairperson of the Yarkuwa Indigenous Knowledge Centre and a Wamba Wamba Pereppa Pereppa Nation woman recalls that in the 1960s mussels were very abundant in the region, probably in their “millions.”

“When my mum would take my kids fishing, she would go out and collect mussels,” says Jeanette. “She taught them how to collect them, how to feel for them with their feet and then duck down in the water and get them.”

Jeanette sees researchers, such as Nicole, coming to talk about their research and share knowledge as “extremely valuable, especially for the younger generation, in terms of their understanding the environment they are living in.”

The Knowledge Sharing Day included an opportunity for people to use a microscope to look at glochidia (the young, parasitic life stage of freshwater mussels) that had attached to fish caught in the Edward/Kolety Wakool River System. The day concluded with a field visit where the rangers saw and discussed the use of side-sonar technology for surveys conducted by Austral Research and Consulting.

Jeanette says collaboration between Traditional Owners and scientists is very important for the future.

“Our main aim is to have our people trained, be exposed to different ways of thinking,” says Jeanette. “In terms of good natural resource management, you need to understand how everything is connected and how the diminishing of one species means that there is something going on, that an imbalance in the environment is being created which then affects so many other things. We need to understand how to deal with a problem when we see a problem developing.”

Our work in the Edward/Kolety-Wakool River system

The Wamba Wamba or Wemba Wemba, and Perrepa Perrepa or Barapa Barapa are the traditional owners of the Edward/Kolety-Wakool River system – a large anabranch of the Murray River in the southern MDB. The system is a complex network of interconnected streams in a productive agricultural landscape, and our work aims to understand how native fish, vegetation, water quality and processes that support and sustain aquatic Food webs, are influenced by targeted environmental watering actions.

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John Trethewie and Jack Hamilton retrieve a Fyke Net from a river.


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