Multi-decadal trends in abundance reveal a dramatic decline in wild Norwegian Atlantic salmon

Summary

Understanding changes in population abundance is critical for conservation of fish populations. Here, we esti mate abundance of wild Atlantic salmon (Salmo salar L.) in Norwegian rivers at national and regional levels using catch statistics encompassing nearly 450 populations. Estimates based on body size were reconstructed nationally (1983–2023) and for five geographic regions (1989–2023). Abundance of one-sea-winter (1SW) and multi-sea-winter (MSW) salmon was reconstructed regionally over 1989–2023. Nationwide abundance halved over four decades, consistent with declines reported elsewhere in the North Atlantic that are often linked to poorer marine growth conditions. The national decline was likely driven by pronounced reductions in the Central and South-West regions and in the River Tana, whereas abundance remained relatively stable in the Northern region and increased in the South-East region. Patterns from sea-age analyses closely mirrored those from size- based analyses. Contrasting regional abundance trajectories provided an opportunity to assess whether observed patterns were qualitatively consistent with independently documented geographic variation in anthropogenic pressures. Sensitivity analyses showed that alternative assumptions regarding unreported catch influenced the estimated magnitude of long-term abundance change but did not alter the direction of national or regional trends. Our results are consistent with reduced marine growth contributing to the nationwide decline in Atlantic salmon abundance, while the marked regional variation in abundance trajectories suggests that regionally varying anthropogenic stressors, including salmon farming, freshwater habitat degradation and human exploitation likely also contribute to finer geographic scale differences in abundance.