Inland Science and Research
European eels spawn in the Sargasso Sea area of the mid-North Atlantic Ocean. After hatching, their larvae drift towards the coasts of Europe where they metamorphose into transparent ‘glass eels’ and, upon entering freshwater, develop into elvers as they migrate upstream. They remain in freshwaters for up to 20+yrs before heading back to the Sargasso Sea to spawn and die.
On a global scale, eels have suffered a massive decline in abundance in recent decades and are currently listed as Critically Endangered by the International Union for Nature Conservation (IUCN). Returns of glass eels have improved in many areas of Europe during the last three years indicating that the decline may have halted and populations may at last be improving. However the International Council for the Exploration of the Sea (ICES) still considers the status of the species to be critical and recommends that anthropogenic impact on stocks is minimised.
Eels are widespread in running and stillwaters on the Isle of Man but data on Manx populations are limited. Two studies conducted by MSc students in recent years indicate that the Isle of Man may have relatively healthy populations, probably due to the lack of commercial exploitation, but also that the recruitment to some rivers may not be sustainable.
Atlantic Salmon and Brown Trout
Undertaken by the Department of Environment, Food and Agriculture’s (DEFA) Fisheries Division, the ‘Juvenile Trout and Salmon Monitoring Programme’ aims to identify trends in the Manx juvenile population, provide an overview of the status of the population in a catchment, and identify those parts of the system that are under performing.
Results for 2020 demonstrated overall that trout populations appear more resilient to adverse factors to Salmonid population growth including climate change, fish barriers among other influences. Salmon populations appear less resilient, requiring management efforts to assist in their survival. Without aid, the future of Atlantic salmon populations in Manx rivers appears under threat. Adult mortality rates at sea for Manx sea trout and salmon are unknown.
Over the past 10 years, brown trout populations remained stable with 80% of monitored sites showing grades within excellent to average densities. Salmon scores within excellent to average densities were slightly under 40% of sites. Brown trout are generally prevalent with classification scores suggesting resilient populations. This may be attributed to trout greater resilience to environmental changes, competitiveness and the ease of access to spawning grounds that resident trout populations obtain without the need to migrate back to and from the sea.
Atlantic salmon juveniles compete for habitat and food with trout which becomes exacerbated further by low water flows. Susceptible to pollution and habitat disturbance, human activity has also adversely impacted juvenile Salmonid survival (i.e. Riparian strip clearance at the NSC in 2014 may have increased the risk of predation). Seriously impacted by barriers, adult fish passage during migration has been further exacerbated by low and high water flows. The concentration of adult Salmonids at barriers make them vulnerable to predators, poachers and anglers alike. High concentrations of spawning adults within limited spawning habitat create further losses if successful redds are disturbed by other spawning fish, worsened by severe spates (sudden flood waters). Removal of fish barriers will improve resilience, opening up a wider catchment area where suitable spawning grounds can be explored.
The assessment of both population trends against management initiatives has demonstrated positive effects from fish passage improvements (e.g. the Raggatt rock ramp installation in 2009) and stocking measures (e.g. Sulby, Santon Burn and Silver Burn). Ongoing negative impacts from historic barriers to fish passage (e.g. Lady Young’s Weir) and climate change (e.g. Storm Desmond in 2015) challenge population resilience.
Marine survival rates are a concern throughout the UK and would therefore appear to be a concern to the Isle of Man. The North Atlantic Salmon Conservation Organization’s (NASCO) ‘Implementation Plan for the period 2019 – 2024’ outlines the threats faced by Atlantic salmon in England and Wales, assessing 44 out of 64 rivers (designated as principal salmon rivers) as at high risk. NASCO states that environmental changes, particularly in the ocean, may be driving this decline, which has required the adoption of stringent management measures on a wide range of pressures confronting the resource, in order to maximise the number of fish returning to rivers to spawn.
Data on the numbers of adult salmon and sea trout returning to Manx rivers would provide a more accurate indication of both the quality and sustainability of the fishery across the life stages. This would also assist in guiding future management strategies.
With the international decline in stocks of Atlantic salmon, sea trout fisheries are becoming increasingly important and valuable. However, as with salmon, sea trout populations in the British Isles have declined in many areas in recent years, sometimes but not in all cases, associated with the proliferation of commercial salmon farming. Between 2010 and 2013, the Isle of Man Government participated as an associate partner in the ‘Celtic Sea Trout Project’ (a multi-agency partnership investigation into sea trout stocks and fisheries of rivers entering the Irish Sea).
Funded by the European Union, the Ireland/Wales Programme with additional support from government agencies, voluntary bodies and private fishery interests, this project sought to inform management recommendations for sea trout fisheries and explore the use of sea trout as a bio-indicator for climate change impacts. The final project report was published in 2019 including information on the Manx fishery estimated an annual return of sea trout to the Island of between 4,154 and 8,308 and the finding that sea trout in the Isle of Man display the highest annual survival rate of the river networks studied (pers. comm. – Dr Nigel Milner, Associate Senior Fisheries Biologist, APEM Ltd).