Abstract
Survival during migration typically depends on body size, in which smaller migrants suffer higher mortality. Thus, migratory animals are predicted to adopt growth tactics to attain large size before migration. Size-dependent growth patterns represent such a case, in which smaller migrants exhibit rapid growth and delay start of migration (extended pre-migration period) to attain a large body size to survive migration. To evaluate adaptiveness of such size-dependent growth patterns, it is crucial to understand costs associated with rapid growth and delayed migration start, since the adaptiveness of the size-dependent growth patterns cannot be solely explained by ecological demands of rapid growth and late migration start. However, potential costs remain largely unknown. Here, we focused on the trade-off between growth and survival, and investigated whether faster pre-migration growth rates and longer pre-migration periods incurred higher predation risk in masu salmon (Oncorhynchus masou), which exhibit size-dependent growth patterns. In a capture-mark-recapture survey examining predation-caused wounds as a proxy for predation risk, we found a non-significant effect of growth rate but a significantly positive effect of timing of migration initiation on the frequency of predation-caused wounds. In particular, migrants that stayed longer in the river had higher probabilities of having predation-caused wounds, especially inflicted by piscivorous birds. This implies that smaller 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.