Costs of size increase prior to oceanic migration inferred from predation-caused wounds in an anadromous fish

Since the mortality during migration is typically greater among smaller individuals, migrants should adopt growth tactics to attain a large size before their migration trip. Size-dependent growth patterns represent such a case, where smaller pre-migrants exhibit rapid growth and delay the start of migration to attain a large body size. To understand factors that shape size-dependent growth patterns, revealing the costs associated with rapid growth and delayed migration is crucial, as size-dependent growth patterns cannot be solely explained by ecological demands of growth. We focused on the trade-off between growth and survival and investigated whether faster growth rates and longer pre-migration periods incurred higher predation risk in pre-migrants of masu salmon (Oncorhynchus masou). A capture-mark-recapture survey was conducted and predation-caused wounds as a proxy for predation risk were checked. Migrants that exhibited higher growth rate did not have higher probability of predation-caused wounds, but migrants that stayed longer in the river had higher probabilities of having predation-caused wounds, especially inflicted by piscivorous birds. This implies that smaller pre-migrants extend their stay in the river to attain larger size for surviving oceanic migration, although the extended stay in the river is costly in terms of increased predation risk.