Availability of holding habitat in lakes and rivers affects the incidence of spring (premature) upriver migration by Atlantic salmon

Abstract

Premature migration is a key component of the biocomplexity of anadromous fishes, yet remains poorly understood. Many Atlantic salmon (Salmo salar) populations leave the ocean in spring, months prior to spawning, though this curtails feeding in productive marine environments. We hypothesized that habitat features encourage the evolution of this trait by providing fish with physical and thermal refuge during their long freshwater holding period. We document substantial variation in Atlantic salmon return-migration timing across 70 Irish rivers using 8 years of angling data, validated with electronic-counter data from 23 of the rivers. A higher frequency of spring migrating salmon was observed in rivers with accessible lakes and larger rivers in general. Spring migration may have evolved in rivers with suitable holding habitat as a strategy to minimize cumulative marine mortality, which is traded off against additional marine feeding. More research on this neglected topic is needed, given the value of large spring migrating Atlantic salmon to anglers, ongoing declines in their abundance in Ireland and elsewhere, and the widespread occurrence of premature migration in salmonids generally.