The marine life of Atlantic salmon: evidence from the chemistry of scales

Abstract

This research provides a new method to identify likely marine feeding grounds for migratory pelagic species that are problematic to directly study at sea. The method is
based on stable isotope compositions of tissues that may be sampled without harming the target animals, and can be conducted retrospectively from tissue archives. The wild Atlantic salmon has been in steep decline throughout its native range over the past four decades, largely due to increases in marine mortality. This research investigated potential causes of this decline using stable isotope analysis of archived scale samples, taken from returning adult salmon over the past few decades. Investigations of UK scale holdings identified the River Frome and Northeast Coast Driftnet Fishery archives as the most available and useful, giving good spatial contrast and temporal coverage.

After developing sampling and analytical protocols, carbon and nitrogen isotopic composition was measured in grilse (one-sea winter) and multi-sea winter (MSW) salmon scale samples taken from both archives over 23 and 14 years. Analyses were performed on the last marine growth season, giving a retrospective record of marine conditions experienced by each fish. Both isotopes are influenced by baseline environmental conditions, and climatic effects are found to exert strong controls on numbers of fish returning to both the Northeast Coast and River Frome populations. Trophic level and/or baseline nitrate effects are also found to influence returning abundance to these populations, although more strongly in the Frome. Yearly d13C values were correlated with median yearly sea surface temperature values for each degree of latitude and longitude across the North Atlantic, and maps produced of the correlation strengths. These maps suggest likely feeding grounds for each cohort within each population, with the River Frome grilse and MSW salmon respectively feeding near the shelf breaks of northeast and southwest Iceland. The Northeast Coast grilse and MSW salmon were, in contrast, feeding near the shelf breaks of the southern Norwegian Sea and the Bear Island Trench in the northern Norwegian Sea, respectively.

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