Tagging freshwater pearl mussels – an update from the FBA Ark
9 October, 2025
Here at the FBA Ark tagging freshwater pearl mussels is an important activity as part of our species recovery programme.
The tagging allows us to assign a unique identification number to each of our mussels, so we can monitor their individual growth over time. We attach the specially designed glue-on Hallprint tags to juvenile mussels before placing them into the FBA flume. They remain in the flume system, under the watchful eyes of the team at the Ark, until the day of their release.
So a little more about the freshwater pearl mussel tagging process
Juveniles with tags are currently used for releases, so that it is possible to distinguish between juveniles bred at the Ark from naturally bred juveniles in the future. Additionally, some juveniles with Hallprint tags are also tagged with PIT tags which can be monitored once juveniles have been released back into the river.
The combination of the PIT and Hallprint tag allows the FBA team to determine which juveniles are still present at release sites over time and trace the individual mussels to specific cohorts bred here at the FBA.
Each population at the Ark is allocated a certain tag colour, for example the Clun is pink, so only captive-bred Clun mussels will receive pink tags. This year we have tagged ≈1,000 Irt and ≈1,000 Clun juveniles with the Hallprint tags, with the help of our dedicated volunteers! The tags are attached to the shells with superglue, causing no harm to the mussels.
What's the difference between the ID tags and PIT tags?
We have the unique ID number Hallprint tags to monitor growth whilst the freshwater pearl mussels are residing at the Ark. The PIT tags are small electronic devices used to track individual mussels' survival after they burrow into the riverbed, following their release back into the wild. The PIT tags enable repeated, non-destructive sampling, improving conservation efforts by aiding individual identification and recapture in low-visibility conditions.
Photos of tagged freshwater pearl mussels at the FBA Ark.
Why it is important to monitor growth rate of the freshwater pearl mussels at the Ark?
Freshwater pearl mussels (Margaritifera margaritifera) grow slowly in captivity, taking 6-9 years to reach sizes suitable for release and approximately 12 years to reach sexual maturity. Their growth rate depends on environmental factors, with captive breeding programmes aiming to release juveniles into suitable river habitats once they are large enough to survive.
“Tagging has been a big priority this year, now that we have a third population for ready for releases into Cumbrian rivers, every spare moment we can dedicate to doing some tagging we have dutifully done so! Some have already been released this year, and others have been tagged in preparation for the busy 2026 field season, where we are hoping to release 2,000 into the Kent alone. Luckily for us this summer we have had the help of volunteers that massively boosted our tagging numbers, sometimes doing several hundred per day.”
Captive breeding and growth of freshwater pearl mussels
Juvenile stage: After their larval stage, the young mussels drop off from the host fish into the Ark’s specially created riverbed and spend several years (around 3-5) developing.
Release size: In captive breeding programmes, the aim is to rear the mussels until they are large enough for release into the wild, which typically takes 6-9 years.
Sexual maturity: Freshwater pearl mussels do not become sexually mature until they are much older, around 12 years of age, sometimes as late as 20 years.
Long-term growth: They are one of the longest-lived invertebrates known, with lifespans exceeding 100 years.
Why captive breeding at the FBA Ark species recovery centre is necessary
Low recruitment: Natural populations of freshwater pearl mussels face significant challenges due to high losses during the larval stage and the requirement for pristine river habitats.
Habitat restoration: Captive breeding programmes, like the one at the FBA's Freshwater Pearl Mussel Ark, aim to boost wild populations by releasing juveniles into rivers where habitats are being actively restored and improved.
Louise Lavictoire and Jodie monitoring tagged freshwater pearl mussels that have been released back into the wild.
Many thanks to all the amazing volunteers who have helping with the tagging of our freshwater pearl mussels at the FBA Ark… we really appreciate your support!
Interested in discovering more?
Find out more about the FBA's Freshwater Pearl Mussel Ark.
Read: Avoiding extinction: Conservation breeding and population reinforcement of the freshwater pearl mussel, 20 May, 2024 by Louise Lavictoire & Chris West.
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.
The Hallprint tags we attach to the freshwater pearl mussels are from the Hallprint company in Australia.
