Annual freshwater pearl mussel census at the FBA Ark
19 March, 2025
The FBA has just completed its annual exhaustive surveys at the Species Recovery Centre (SRC) in Windermere.
This is a thorough assessment of all freshwater pearl mussels in our captive breeding facility that takes place every winter, when the water is less than 10°C and the mussels aren’t growing as much. The aim of this comprehensive census is to capture critical information about the development of the mussels at the FBA Ark, to gain a complete and accurate picture of the health of the population.
Here we join our River Kent Project Officer, Samantha Bonny, to find out more about this significant annual census.
What is required for the annual exhaustive survey of the Ark’s freshwater pearl mussels?
It involves counting individual mussels that are more than 5mm in length and reporting these numbers back to partners.
The Ark is rearing mussels in different systems including heated aquariums, gravel trays, upweller tanks and flumes, as well as tanks containing our broodstock adult mussels. All these mussels must be individually counted (Pictures 1 and 2), which is a slow but critical process, allowing an accurate record of how well mussels are surviving and developing in the different rearing systems, and between populations.
At the Species Recovery Centre, the FBA has finished its annual "Exhaustive Surveys", a census which involves counting all mussels in the facility that are more than 5mm in length. Counts take place on every mussel rearing system and from this a representative sample will be taken to determine average lengths of the freshwater pearl mussel population at the FBA Ark.
In total, over 32,000 mussels were counted in 2025, with several thousand having been released into rivers in the field season of 2024.
Photo (left) shows Jodie conducting an exhaustive survey on a flume system, digging through the substrate to extract and measure as many mussels as she can find. Photo (right) of the mussels found in the flume survey, the individual in Jodie’s hand shows visible shell growth rings.
Tracking mussel growth at the FBA Ark
As well as counts, the FBA wants to track the growth of mussels. For systems with a small number of individuals (less than 100), such as the broodstock tanks which hold around 30 adults, we can measure every mussel. With mussels larger than 8mm, we tend to use callipers as shown in the photo below. The calliper jaws should be gently touching the shell tips so that the mussel is held securely, without placing any stress on the shell.
If a mussel is tagged with a vinyl number tag, we can closely track the growth of that mussel over time, doing repeated measurements each year, as well as when it moves between systems and even out to a river when it is released. This provides a really useful indicator on the productivity of each system and allows us to predict when mussels will reach a size large enough for reinforcement of wild populations (more than 15mm).
Photo (left) of a mussel with a vinyl number tag being measured using metal callipers. Photo (right) part of a sample group of mussels in a petri dish of water, measured from a tray rearing system. The callipers at the top act as a scale. The labels indicate the number of mussels, the population and the date.
The photo of the sample group of mussels can then be used in a software called ImageJ, which allows us to accurately measure many mussels more quickly (shown in the photo below). Using this method also means that we can measure several cohorts in a row, whenever staff have time, which rapidly speeds up the process and provides flexibility.
Image of a sample of mussels in a petri dish, loaded into the software ImageJ. Yellow lines are drawn along the mussels’ length and automatically produce a measurement, as seen in the table centre right.
“I’m delighted to have completed the Exhaustive Surveys for 2025 and reported back to our partners and contributors. It was a big joint effort from Jodie, Ben, myself and our volunteer Sarah and at times we had very cold fingers, but it was worth it to reveal nearly 32,000 juvenile mussels thriving in our facility.”
Interested in discovering more about freshwater pearl mussels at the FBA Ark?
Read: Avoiding extinction: Conservation breeding and population reinforcement of the freshwater pearl mussel, 20 May, 2024 By Louise Lavictoire & Chris West
Find out more about FBA's Freshwater Pearl Mussel Ark
Established in 2007, the Freshwater Pearl Mussel Recovery project is an ongoing partnership project between the Environment Agency, Natural England and the Freshwater Biological Association.
