The River Ehen is situated in Ennerdale, West Cumbria and flows from Ennerdale Water, discharging 27 km downstream into the Irish Sea at Sellafield.

The River Ehen supports the largest viable population of freshwater mussels (FWPM) left in England. The majority of which are adults.

The River Ehen is designated as a European Special Area of Conservation (SAC), designated for FWPM and Atlantic salmon and also as a Site of Special Scientific Interest (SSSI) for FWPM. The SSSI has been assessed by Natural England as being in “unfavourable declining” condition due to insufficient freshwater mussel recruitment, making the current population unsustainable.

Surveys indicate that juvenile mussels are present but they are rare and not enough are surviving to offset mortality. The population is therefore ageing and declining and urgent action is required to prevent extinction of this critically endangered population.

Mussels are declining rapidly internationally and are listed as critically endangered, equivalent to the risk of extinction of the Orang-utan and Gorilla. In the last 90 years, they have declined globally by 61.5% and by 87% in Europe. As Britain (mainly Scotland) contains a significant quantity of the known breeding populations of FWPM in Europe, the Compensatory Measures Project has significant implications for the whole of the Europe. Given the importance of the British populations in a European and global context, their loss would have a catastrophic impact on the overall survival of the FWPM in Europe. The importance of the River Ehen mussel population can therefore not be overstated

The River Ehen is principally a migratory salmonid river. Although catches of salmon and sea trout have been very good historically, current catch levels are now significantly lower than previously recorded and runs of adult salmon back into the river are of prime concern at present (2018). The underlining issue with salmon stocks is very poor marine survival which appears to be having a disproportionately negative impact on one-sea-winter “grilse” which have historically formed the bulk of the River Ehen salmon run. This decline is also of concern with regard to the mussel population as they are dependent on salmon as part of their lifecycle.

There is also a small population of resident brown trout in the River Ehen, while Ennerdale Water and its tributaries contain an important population of brown trout and the most significant extant population of river-spawning Arctic charr in England and Wales.

The EA requires that all salmon and sea trout anglers return detailed records of their rod catches each year.These rod catch data include all rod caught salmon and sea trout along with whether or not they were released back into the river.

The River Ehen salmon population is currently considered to be “Probably At Risk” and is predicted to remain so in five years’ time, based on an assessment of estimated egg depositions over the last ten years (2007-2016). These data show that the stock as a whole has remained above its Conservation Limitin most years but fell below it in 2015 and only just exceeded it in 2016. The overall trend in salmon egg deposition in the River Ehen is now downwards.

Conservation Limit is the minimum level of egg deposition that is desirable in a catchment in order to maintain it at a sustainable population (catch) level. This limit is based on an assessment of the naturally available spawning habitat within the Ehen catchment including all relevant tributaries. Annual egg deposition estimates are then calculated from rod catch data each year and compared to this Conservation Limit to determine how healthy the overall population is.

Some of the reasons for the decline of both species include habitat degradation from nutrient run-off, excess silt covering the riverbed and an unfavourable flow regime. The Compensatory Measures Project will be delivering conservation projects to address these issues.