Sam Jones – River Esk Project Student
I thought I would take a little time to mention our Freshwater Pearl Mussel encystment work this summer (part of the Esk Pearl Mussel and Salmon Recovery Project). The mussel is a large long lived mollusc that inhabits a small number of rivers in the United Kingdom including the River Esk in the north of the Park. The species is endangered and struggling to reproduce naturally within the Esk, resulting in an aging population with falling numbers.
The mussels reproduce by releasing millions of microscopic young (called glochidia at this stage) into the open water in late summer. A lucky few of these glochidia drift onto the gills of young salmon and trout fry or parr in the river. These young latch onto the gills of the fish and grow there over winter, causing no harm to the fish. They then, after having grown to about the size of a pin head, drop off around May time and settle in the river bed.
This is natural encystment but as you may guess the likelihood of the glochidia landing on the gills of fish is exceedingly low; the river is a big place. As we have so few mussels, the glochidia need a helping hand in reaching their target. The idea is that this summer we will gather as many fish as possible by electro-fishing (using electricity to stun and catch small fish), and put them in containers on the bankside along with collected fertile female mussels. The mussels will then be encouraged to release their young, and if all goes well we will end up with a high rate of encystment.
We have already been out on the river preparing for this work. In April a few of the local Environment Agency fisheries team were good enough to come along and electro-fish stretches of the river around our planned re-establishment sites. This turned up some good healthy fish populations, but unfortunately we did not see any natural encystment on the gills of the fish this year. It is likely that some natural encystment is occurring in the river, but at such a low level that it is very hard to spot. These results highlight even more the pressing need for the artificial encystment work. We’ll be using electro-fishing again next year to measure how effective this year’s encystment work is, comparing this year’s results with next year’s.
So, over the next month Simon Hirst (Esk Project Officer) and I will be trained up in glochidia monitoring by an expert. Then we will start our mussel checks, looking for fertile females. The unpredictable molluscs have to be closely monitored as they can release their glochidia anywhere from late July to mid-September (likely influenced by temperature, but not definitely known). We will select 20 or 30 female mussels and return to them repeatedly to open them up ever so slightly so we can take a small sample of the developing young. The stage that these young are at will tell us approximately how long until they are ready to be released by the female mussel. It’s all going to be a bit frantic on the actual encystment day as we gather together the Environment Agency’s fisheries team, the fish and the mussels at short notice.
This is one element of a two pronged efforts to save our mussels. There is a Pearl Mussel Ark facility at the Freshwater Biology Association’s Headquarters at Windermere in the Lake District. They have mussels from rivers all over the country including the Esk, and they are attempting to breed them in captivity for re-introduction to the wild. Some progress has been made, but unfortunately as yet, no Esk baby mussels have survived long enough to be brought back to the North York Moors. Hopefully though, if this encystment project goes well and is worth repeating next year, and if the Ark facility can produce surviving young, the Freshwater Pearl Mussels in the River Esk can thrive again!