In October, FBA staff visited the Cynrig and Clywedog Hatcheries operated by Natural Resources Wales. At these sites, NRW conducts captive breeding programs for aquatic species that are critically endangered; these include arctic charr, white clawed crayfish and water voles. In addition to these projects, Cynrig and Clywedog are the locations of two out of the four breeding programs for freshwater pearl mussels (FPM) in the UK. Part of this is funded through the Four Rivers for LIFE project, that shares similar river restoration objectives to the LIFE R4ever Kent project, of which the FBA is a partner organisation. Therefore, this trip was an invaluable opportunity for our Species Recovery staff for knowledge exchange and cooperation between the FBA and NRW, to continuously improve the success of UK FPM propagation.
The Welsh hatcheries breed from Welsh FPM populations exclusively. The purpose of their captive breeding is to release FPM juveniles into suitable river habitat that has been identified through in-river surveys and substrate checks, to boost declining wild populations with reproductively capable individuals. Both facilities use water from natural sources, that is gravity fed through filters and UV-sterilised before being pumped through the systems containing mussels.
Following the same practices as the captive breeding program at the FBA’s Species Recovery Centre, wild-fertilised adult mussels are brought to the Clywedog facility from their respective rivers, where the encystment process can begin. While most Welsh mussel populations use brown trout as their compatible host fish, some are also exposed to Atlantic salmon for encystment, such as the Dee. The gravid female mussels release glochidia into the water column, before it flows into a separate tank of fish. Once fish are well encysted with glochidia, they are transported to the Cynrig hatchery where staff focus on excystment and early-stage juvenile mussel rearing.
At Cynrig, host fish are placed into heated recirculating systems that run at a temperature of 17 degrees Celcius, to accelerate the drop-off process. This accelerated excystment process has been trialed in the past few years at the FBA, and now a new, purposely built system is being built that has capacity for more fish, which will maximise on juvenile collection. Again, like the FBA, NRW uses algal nets attached to collection bottles over the tank outflow pipes for collecting juvenile mussels that have excysted from the host fish, and these very small juvenile mussels are hand-picked using pipettes from underneath a microscope before being introduced to a rearing system.
A significant difference between propagation at the Welsh and FBA hatcheries is that instead of using heated aquaria with gravel-filled sieve cups like the FBA, the Welsh facility utilises incubation tubs for growing newly excysted juveniles. These are highly simplistic plastic airtight containers that are maintained in an incubator at around 16 degrees Celcius, for several weeks. Around 500 mussels are placed into each tub, which is then fed with industry standard algae, as well as a locally sourced detritus material. The tubs require water changes and cleaning every two days, meaning much of NRW staff time is spent on maintenance of this system. Once large enough, the juvenile mussels are moved back to the Clywedog facility where they will continue to grow in trays containing clean gravel, inside a flow-through tank system. The water is no longer heated, and mussels feed on algae naturally occurring in the river, making this next stage much less labour intensive, and allows the juveniles to become accustomed to a habitat that simulates the river environment.
One FPM population – the Welsh River Dee – has unfortunately not produced viable glochidia in a number of years, potentially due to their origin. For many years, these mussels were located downstream of a reservoir where they experienced continuous stressing from flow releases, temperature shocks, drying episodes, and more. Given that the temperature profile of Windermere is consistently warmer than that of Clywedog reservoir, it was decided that upon their return, FBA staff would bring 25 adult River Dee mussels to the SRC. It is hoped that with different water conditions the FBA will be successful in producing healthy, viable juveniles from this endangered population of mussels for the river Dee in the future.
Overall, the trip was a brilliant occasion for FBA staff to learn about the different tank systems utilised at the two facilities and highlighted the need for staff working in this sector to meet regularly to share our successes as well as our hardships, and get advice from one another on our unique FPM rearing systems and captive breeding techniques.
