FRESHWATER PEARL MUSSELS IN THE RIVER ESK

The Freshwater Pearl Mussel (Margaritifera margaritifera) was once abundant in the rivers of Britain, but is now in serious decline. The only river in Yorkshire supporting these large, bivalve molluscs is the Esk, where steps are being taken to establish a breeding population.

The problem is that the life cycle of pearl mussels is ludicrously complex. Firstly, in June or July the males release sperm into the water in the hope that it will be inhaled by a female. Once fertilised, the eggs grow into larvae known as glochidia, and from July to September these are released into the water in huge quantities. The future of these proto-mussels relies on them being inhaled by a salmon or a trout. As they are filtered through the fish's gills along with the river water, they snap onto the gill filaments. The fish then creates a cyst around these tiny hitch hikers. They grow over winter happily encysted on their host's gills, until in May or June the following year they drop off. They need to land on clean, well oxygenated gravel to continue developing into adults. 

With so many variables involved in maintaining this delicate cycle, is it any wonder that the freshwater pearl mussel is now included on the IUCN Red List of Endangered Species? Historical pearl fishing, siltation, pollution and the decline of the host fish population have all contributed to the disruption of the delicate balance required for these important creatures to flourish.

The pearl mussels in the River Esk are the last surviving population in Yorkshire, and only a few mussels are left. The vast majority of the remaining pearl mussels are aged 60 years+, and the mussels in the Esk have not produced young for over 25 years, it is likely that the Esk population will become extinct in the next 40 years unless action is taken to halt this decline. From the North Yorkshire Moors National Park official blog.

Some mussels from the Esk have been taken to attempt captive breeding at a  Freshwater Biological Association 'ark' in the Lake District. There fertilised female mussels are kept with fish in the hope that encysting of glochidia will take place, after which the mussels are removed to a different tank. The introduction of viable mussels back into their host rivers is the projected end result of this work. Of course this will only be possible if the water quality and substrate are suitable.

The freshwater pearl mussel can live for 130 years, so it's quite conceivable that mussels living in the Esk now were around when Frank Meadow Sutcliffe was snapping away.

Further information is available from the links below.

The River Esk Pearl Mussel and Salmon Recovery Project
The Freshwater Biological Association Pearl Mussel Project
Pearl Mussel Videos from Lousie Lavictoire