More on the mussels

Simon Hirst – River Esk Project Officer

As mentioned previously the River Esk in the north of the National Park contains Freshwater pearl mussels (Margaritifera margaritifera). It’s the only river in Yorkshire with the species which is internationally important and classed as endangered. The mussels left in the Esk are nearly all over 60 years old, which means they are on the verge of extinction here. The National Park Authority is leading on the Esk Freshwater Pearl Mussel and Salmon Recovery Project to try and secure a future for the species in this region.

The lifecycle of the freshwater pearl mussel is very complex and unusual. And really quite interesting.

The male pearl mussels release sperm into the water, which is then filtered by the females. The eggs are fertilised, and develop in the female’s breeding pouch before being released as larvae. Each female mussel can release up to 4 million larval mussels. These larvae (also known as glochidia) are then washed downstream by the flow of the river, and if they are lucky they attach themselves to the gills of young salmon or trout.

The young mussels live as a parasite on the gills of the fish for approximately nine months, before dropping off the fish and settling into the river gravels.

They then grow in the river bed and develop into juvenile pearl mussels. During this time they are very sensitive to pollution and sedimentation (smothering).

The juvenile mussels take 12-15 years to reach maturity. The mussels can live for over 100 years.

So what are we doing to try and help?

Whilst Freshwater pearl mussels from the Esk are breeding at the Captive Breeding Facility in Windermere, we’re working here to improve the river and riparian habitats so that our mussels can be released back into their natural environment.

This means tackling the sedimentation and nutrient problems in the river and its tributaries. It means working with Universities to study the effects of water quality. It also involves getting local people of all ages interested in the future of the mussels and the health of the river in the long term. The mussels need salmon and trout to act as hosts and salmon and trout benefit from the watercourse improvements as much as the mussels. Salmon and trout are also a valuable local resource for the local economy.

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