Assessing Trends and Challenges: Insights From 30 Years of Monitoring and Management of Threatened Southern Atlantic Salmon Populations

Abstract

The Atlantic salmon (Salmo salar) has suffered significant population declines worldwide, prompting urgent conservation efforts, especially in its southern distribution area. This study is aimed at characterising the population dynamics of Atlantic salmon in the Bidasoa River (Spain), by focusing on fluctuations and long-term trends in salmon returns, population characteristics and effects of angling and stocking activities. For this, monitoring data spanning three decades (1993–2023) from a salmon monitoring station and anglers' captures were used together with data on stocking activities provided by the Fish Management Section of the Navarre Government. Results reveal cyclical patterns driven by three distinctive wavelengths linked to the salmon life cycle, climatic variations and local habitat and connectivity improvements. The Bidasoa population was primarily dominated by one-sea-winter males returning during the autumn–winter season. Over the study period, a significant reduction in body size was observed, likely reflecting challenging marine conditions. Angling pressure was notably skewed towards multi-sea-winter females, which could influence natural recruitment dynamics. Despite the low mean stocking return rate of stocked salmon (0.13%), they contributed to one-third of the annual returns, underscoring their potential role in supporting population persistence albeit at the expense of reducing natural spawning, amid broader ecological challenges. This study provides valuable insights into the complex interplay of ecological and anthropogenic factors affecting Atlantic salmon populations in southern European rivers. These insights are crucial for developing and implementing effective conservation strategies aimed at preserving the Atlantic salmon, a species of significant cultural and ecological importance.