New Missing Salmon Alliance Paper Brings us Closer to Understanding the Decline in Marine Survival of Wild Atlantic Salmon

Wild Atlantic salmon have declined by around 70% across the North Atlantic over the last few decades. In Great Britain the species was listed as endangered by the International Union for the Conservation of Nature (IUCN) in December 2023, with British salmon having suffered an additional 30-50% decline since 2006 and a projected 50-80% decline between 2010-2025. Whilst the species faces many pressures in freshwater, in the coastal zone and at sea which are all having an impact, recent declines have been driven primarily by a dramatic reduction in marine survival. Simply put, of the juvenile salmon which leave their rivers and head out to sea to grow and mature, fewer are surviving to come back as adults.

The Missing Salmon Alliance team has now published a scientific paper which links this drop in marine survival to declining zooplankton energy through the juvenile salmon food chain. Zooplankton is essential prey for the larvae of fish such as sandeel and blue whiting which young salmon ‘post-smolts’ (fish which have entered the marine environment) rely on for feeding during their marine life phase.

The study found that ‘zooplankton prey energy has significantly and dramatically declined over much of the northeast Atlantic, and specifically within key salmon migration domains, over the past 60 years.’

It concludes that zooplankton energy could be a useful indicator of marine return rates for some groupings of southern European wild Atlantic salmon populations, and a more consistent positive relationship with post-smolt survival than previously investigated indicators based on ocean temperature and primary production. Further work is looking at the drivers causing zooplankton declines and the consequences for wild Atlantic salmon populations.

The analysis included wild Atlantic salmon data from rivers around the British Isles and northern France, including the Bush in Northern Ireland, Corrib and Burrishoole in The Republic of Ireland, Tamar and Frome in England, Dee in Wales, North Esk in Scotland, and Bresle and Scorff in France, to build a regional picture.

This work feeds into the Alliance’s flagship project, the Likely Suspects Framework data model. This aims to quantify the impact of various factors influencing wild salmon abundance at each stage of their lifecycle, enabling restoration and management efforts to be targeted at the right lifestages for maximum conservation impact.

While more work must now be done to further understand the drivers behind low marine survival, including the impact of fisheries bycatch, the paper reaffirms one of the Alliance’s key work areas in freshwater – the restoration of river catchments to allow young salmon to head out on their ocean migration in the best possible condition.

Studies have shown that the larger and fitter juvenile wild salmon ‘smolts’ are when they leave freshwater and head out to sea, the better their chances are of returning home as adults. By combining efforts across research, restoration, advocacy and the law, the Alliance is working together to energise catchment-scale restoration and better protection for the species across the country. These efforts focus on giving young wild Atlantic salmon the best start in life - access to cold, clean water in climate resilient and biodiverse landscapes, maximising feeding and growth so they have the best chance of overcoming the challenges which exist at sea.