Salmon eyes may hold secrets of their survival … and decline

Scientists are investigating a new method to try to understand why UK Atlantic salmon populations are in such drastic decline – by looking deep into their eyes.

The iconic Atlantic salmon are anadromous fish which means they begin their life in freshwater streams and rivers before migrating to the ocean to grow and mature.

Researchers from the University of Essex are joining forces with the Missing Salmon Alliance, the University of Southampton and other river biologists from England, Scotland and Ireland to try to get a better understanding of how well the freshwater phase of their life cycle prepares them for the gauntlets they will face in the ocean.

They will do this by investigating tissues which grow throughout the fish’s lifetime – in this case the eye lenses and the ear stones (otoliths).

Project lead Dr Anna Sturrock, for Essex’s School of Life Sciences, said: “This iconic fish has experienced unprecedented declines in recent years and because they spend part of their life in freshwater and part of their life in the ocean they are vulnerable to a wide range of stressors.”

She hopes this project will give them a better insight into the critical habitats supporting these fish and to understand the factors driving their growth and migration behaviour by investigating the layers of the eye lenses and otoliths. These will allow the team to “look back in time” to identify which freshwater and marine habitats were used by different populations to help inform managers trying to protect critical habitats to improve salmon growth, health and survival.

“There are a growing number of excellent studies exploring fish movements using electronic tags,” added Dr Sturrock, “but I am excited to complement these by analysing natural tags in tissues that are constantly recording information about the environment the fish is growing in, such as its diet and the local water chemistry. Natural tags allow us to track the movements, metabolic rate, diet and growth of fish across their lifetime, including species or life stages too small to tag externally, allowing us to build a more comprehensive picture of the factors influencing fisheries productivity and stability.”

Dr Colin Bull, of the Missing Salmon Alliance, added: “We are delighted to be working with this group of scientists and supporting new work in this area. With wild Atlantic salmon stocks in such a mess we need novel research like this to seek answers and provide essential guidance on how to maximise the survival chances of this species.”

For now they are focusing on returning adults but future studies will sample the same populations and cohorts at different points across their lifecycle to see which fish in the juvenile sample disappear from the adult sample to identify the factors that have the largest effect on survival.

Dr Sturrock and collaborators at the Center for Watershed Sciences at the University of California, Davis, have been developing these techniques for Chinook salmon in California, where the data show that in an increasingly unpredictable climate, the “winners” can vary considerably among years, indicating the importance of life history diversity and risk-spreading.

“Ultimately, we hope that these approaches will complement the tagging data and other forms of monitoring data to help us better understand why so many salmon go ‘missing in action' each year,” added Dr Sturrock.

Helping with the sampling are the: Cromarty Firth District Salmon Fisheries Board, Kyle of Sutherland District salmon Fisheries Board, Deveron, Bogie Isla, Rivers Charitable Trust, Findhorn Nairn and Lossie Fisheries Trust, Game and Wildlife Conservation Trust and Marine Institute, Ireland.