Linking declining Atlantic salmon (Salmo salar) populations with multidecadal changes in their marine foraging ecology

Abstract

Atlantic salmon (Salmo salar) populations have declined considerably, largely attributed to low marine survival linked to large-scale changes in oceanographic conditions. Yet the link between marine environmental change and Atlantic salmon growth, diet, and distribution remains poorly understood. Using stable isotope analysis of archived scales, this thesis examines spatial and temporal trends in salmon marine resource use and trophic ecology over the last 50 years.

Carbon and nitrogen isotope ratios of salmon scale collagen revealed significant shifts in marine resource use by salmon foraging off West Greenland, coinciding with major ecosystem regime changes in the Northwest Atlantic. Amino acid δ15N-based trophic position estimates remained relatively stable through time, though high within-year variability suggests diverse foraging strategies among individuals. These results indicate that, although salmon trophic ecology has not undergone major secular shifts through time, salmon are likely influenced by bottom-up controls and prey availability.

Comparisons of δ13C and δ15N from returning adults showed that one-sea-winter (1SW) and multi-sea-winter (MSW) salmon from the Outer Bay of Fundy and Southern Upland populations occupy distinct summer foraging areas, whereas both life histories from the Inner Bay of Fundy share a common but separate area. Isotopic enrichment of returning salmon and individuals caught in the Northwest Atlantic further supports population-specific differences in foraging. Correlations between δ¹³C and sea surface temperature offer promising insight into foraging locations, though their utility remains limited in coastal environments.

Growth rate estimates from scale measurements revealed a long-term decline in marine growth, most evident in the first marine summer and varying across regions and life histories. In addition to temporal trends, spatial and individual-level variation highlighted trade-offs between freshwater size and early marine performance, including evidence of compensatory growth. Divergent growth trajectories between 1SW and MSW salmon point to distinct resource allocation strategies and potential genetic influences.

Overall, this work highlights the value of long-term biological archives and stable isotope approaches for assessing migratory species’ responses to shifting marine ecosystems. The findings demonstrate that Atlantic salmon do not respond uniformly to environmental change, underscoring the importance of conservation and management strategies tailored to specific populations, life histories, and regions.