Adult return rates of the anadromous Atlantic salmon: influences of smolt length across a latitudinal gradient.

Abstract

Populations of anadromous Atlantic salmon Salmo salar (‘salmon’) have decreased dramatically across much of their geographic range in recent decades. It is posited this is being driven by low marine survival, resulting in reduced adult return rates to natal rivers. We hypothesised that marine return rates (MRR) of adult salmon are positively influenced by their body lengths as seaward-migrating juveniles (‘smolts’), so larger smolts will be more likely to survive at sea and return as spawning adults to the natal river. We also suggest that the magnitude of this effect decreases with latitude, with its effect being weaker in more northerly populations. These were tested using individual capture-mark-recapture data from seven salmon populations in Western Europe spanning 13 degrees of latitude and 12 years of smolt cohorts. We used a multi-state state-space model to test whether MRR was influenced by smolt body length and latitude, and their interaction. Across the seven populations, smolt length had a significant positive effect on the probability of adult return, which was stronger in southern versus northern populations. Atlantic salmon smolt body length is thus an important indicator of their MRR, especially among populations in the warmer, southern part of their geographic range, with this having important implications for conserving declining salmon stocks. It suggests that management in their freshwater juvenile stages can increase MRR and should focus not only on the numbers of seaward-migrating juveniles, but also their individual quality, as indicated by their body length.