Home ground advantage: Local Atlantic salmon have higher reproductive fitness than dispersers in the wild

Abstract

A long-held, but poorly tested, assumption in natural populations is that individuals that disperse into new areas for reproduction are at a disadvantage compared to individuals that reproduce in their natal habitat, underpinning the eco-evolutionary processes of local adaptation and ecological speciation. Here, we capitalize on fine-scale population structure and natural dispersal events to compare the reproductive success of local and dispersing individuals captured on the same spawning ground in four consecutive parent-offspring cohorts of wild Atlantic salmon (Salmo salar). Parentage analysis conducted on adults and juvenile fish showed that local females and males had 9.6 and 2.9 times higher reproductive success than dispersers, respectively. Our results reveal how higher reproductive success in local spawners compared to dispersers may act in natural populations to drive population divergence and promote local adaptation over microgeographic spatial scales without clear morphological differences between populations.

Time spent in distinct life history stages has sex‐specific effects on reproductive fitness in wild Atlantic salmon

“…Adults were weighed and total length was recorded. Condition was calculated as the residual from a linear model of weight predicted by length for each sex and spawning cohort (Mobley et al, 2019a; Patterson, 1992). Scales were collected for age analysis and a small piece of anal fin was collected for genetic analysis before release near the site of capture.…”

“…These individuals that spend less time in freshwater generally spend more time at sea and thus attain sexual maturity later before returning to rivers to spawn (Erkinaro et al, 2019; Jonsson & Jonsson, 2011; Randall, Thorpe, Gibson, & Reddin, 1986). Spending more time at sea has direct reproductive fitness consequences as larger body size is related to higher fecundity (i.e., mature eggs) in females (Heinimaa & Heinimaa, 2004) and higher reproductive success in both males and females (Fleming, 1998; Mobley et al, 2019a). However, spending more time at sea may come with a high cost to survivorship as fewer older individuals return to mate, presumably due to high predation at sea (McCormick et al, 1998; Thorpe, 1994).…”

“…To date, few studies have investigated how time spent at discrete life history stages affects reproduction in Atlantic salmon. Previous studies have mainly focused on sea age rather than the potential for freshwater age to influence reproductive fitness (Mobley et al, 2019a). This is probably due, in part, to the relatively long reproductive cycle of Atlantic salmon and the low survivorship to sexual maturity in natural populations.…”

“…Garant, Dodson, and Bernatchez (2001, 2003) also found evidence for a relationship between body size, reproductive success and mating success in Atlantic salmon in 1 SW and 2 SW males and 2 SW females, but did not characterize freshwater age, rendering comparisons between relevant life history stages incomplete. Building on more recent developments in genetic techniques, sex‐specific life history trade‐offs can measure the reproductive success of spawning adults by reconstructing pedigrees over multiple years (Christie, McNickle, French, & Blouin, 2018; Mobley et al, 2019a).…”

Evolutionary stasis of a heritable morphological trait in a wild fish population despite apparent directional selection

“…For example, Atlantic salmon in the River Teno/Tana in Finland/Norway exhibit a much larger range of male body sizes, and selection for larger males is known to occur there (Mobley et al, 2019; see also Fleming, 1998). For example, Atlantic salmon in the River Teno/Tana in Finland/Norway exhibit a much larger range of male body sizes, and selection for larger males is known to occur there (Mobley et al, 2019; see also Fleming, 1998).…”