Pearl Mussel and Salmon Recovery Project

The freshwater pearl mussel is one of the longest-lived invertebrates known, and can live for more than 100 years. Formerly widespread and abundant in England and Wales, its numbers have severely declined with most former populations now virtually extinct.

The decline in pearl mussel populations is due to a number of factors:

• Habitat degradation caused by sedimentation of river gravels.
• Decline in populations of host fish (salmon and trout).
• Water quality issues.
• Historic Pearl Fishing

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 40 years, it is likely that the Esk population will become extinct in the next 30 years unless action is taken to halt this decline.

Pearl mussels are a very important “indicator species”, which show us the health of our river systems. When species like this start to disappear, then we know that all is not well with the river eco-system.

A Freshwater Pearl Mussel

Image - A Freshwater Pearl Mussel (Margaritifera margaritifera) found in the River Esk. This mussel is approximately 40 years old.

The Lifecycle of the pearl mussel

The lifecycle of the freshwater pearl mussel is very complex and also very unusual. 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. The juvenile mussels take about 15 years to mature, and during this time they are very sensitive to pollution and sedimentation.

Life cycle of the Freshwater Pearl Mussel by David Geist (2005)

Diagram - Lifecycle of the freshwater pearl mussel, image courtesy of Dr Geist (2005).

Funding

Funding for the project has been obtained from a number of sources including the Heritage Lottery Fund, Yorventure, Environment Agency, LEADER and the North York Moors National Park Authority.

River Restoration Work

This funding will be used to carry out habitat restoration work along the River Esk that will improve conditions for pearl mussels, fish populations and a whole host of other riparian species such as Otters, Dippers, Kingfishers and river invertebrates.

So far we have carried out River restoration work on a total of 26 farms, in order to help restore the habitat of the River Esk. Here is a summary of what we have achieved with the funding:

• Installation of riverbank fences to allow “assisted natural recovery” of the river. The fences will prevent bank erosion by livestock and allow a natural buffer strip of native flora to establish (24,000 metres)

• Creation of buffer strips between the river and agricultural land to reduce sediment and nutrient run-off (2-10 metres of land on each side of the river)

• Broadleaved tree planting to create a riparian woodland corridor (1200+ broad leaved trees).

• Improvements to cattle crossing points (using culverts and concrete sleepers) to prevent nutrient and sediment input to the river system. Construction of cattle crossing points (7 sites).

• Provision of alternative watering supplies for livestock (pumps, spring connections and troughs) (18 new installations).

• Management of bankside vegetation including, coppicing and pollarding of dying trees and removal of trees that are causing bank erosion. (Total of 8 days work).

• Gate improvement works next to watercourses (6 sites).

• Invasive plant treatment (Himalayan Balsam and Japanese Knotweed)

Captive Breeding Programme

In September 2007 some of the Esk pearl mussels were relocated to a captive breeding facility in the Lake District.

The Esk mussels bred successfully in Summer 2008 and approximately 200,000 larval mussels encysted on the gills of the host fish at the captive breeding facility.
Approximately 2,000 of these survived and were collected as post-parasitic juveniles. These juveniles are now being reared at the facility.

It is hoped that these juvenile mussels will be re-introduced to the River Esk in around five years time when the habitat in the River Esk has improved.

Larval mussels on the gill filaments of a fish (taken under a microscope)