Linking estuary rearing habitats to migration traits, behaviour, and survival of juvenile coho salmon (Oncorhynchus kisutch)

Estuaries are nursery habitats for juvenile Pacific salmon, yet quantifying estuary contributions to behaviour, adaptations, and fitness across the salmon life-cycle remains a challenge. Using data from 64 coho salmon populations ranging from California to Alaska, I first examined how juvenile migration traits covary with estuary and freshwater habitat characteristics. Populations from lower latitudes, and with access to more enclosed and complex estuaries, exhibited earlier and more protracted migrations. Second, I used a mark-recapture study to examine estuary residence, growth, and subsequent marine survival by juveniles from the Koeye River in British Columbia. Relative to larger individuals, small juveniles resided longer and grew more in the estuary before marine entry. Marine survival increased with juvenile body size and estuary growth improved survival prospects by 44-46%. This research reveals that juvenile salmon are locally adapted to estuary habitats, and that condition-dependent stopover can mitigate freshwater carryover effects and boost marine survival.