Seasonal ecosystem linkages contribute to the maintenance of migratory polymorphism in a salmonid population

The influence of resource subsidies on animal growth, survival, and
reproduction is well understood, but their ultimate effects on life
history have been less explored. Some wild species have a partially
migratory life history, wherein migration is dictated based upon threshold
traits regulated in part by the seasonal availability of resources. We
conducted a large-scale field manipulation experiment where we provided a
terrestrial invertebrate subsidy to red-spotted masu salmon. Individuals
in stream reaches that received a subsidy had, on average, a 53% increase
in growth rate relative to those in control reaches. This increased growth
resulted in a greater proportion of individuals reaching the threshold
body size and smolting in the autumn. Consequently, 19–55% of females in
subsidized reaches became migratory, whereas 0–14% became migratory in the
control reaches. Our findings highlight seasonal ecosystem linkage as a
key ecosystem property for maintaining migratory polymorphism in partially
migratory animals.