Factors influencing fallback by adult Atlantic salmon following transport into a novel river reach

Abstract

Transport of salmonids allows upstream migrating adults to bypass barriers to migration, for example hydroelectric dams or adverse habitats. Downstream movement (“fallback”) after transport is common and removes individuals from spawning populations, with possible detrimental consequences for population productivity. Time-to-event analysis was used to determine effects of fish characteristics, transport conditions and environmental variables on fallback by adult Atlantic salmon transported to an inaccessible river reach as a population enhancement strategy on the Rivière Sainte-Marguerite Nord-Est (Québec, Canada). Of 68 salmon transported, 19 exhibited post-transport fallback within seven days of transport. Fork length (FL) was the only factor that increased risk of post-transport fallback; there was a 5% increase in the proportion of large salmon (≥780 mm FL) exhibiting post-transport fallback relative to individuals of median size (750 mm FL). Although the present study was limited by small sample size, the findings will help inform design of transport programmes in freshwater systems.