State‐dependent estuary stopover boosts juvenile salmon growth: Implications for marine survival

Stopover habitats allow migratory animals to bolster energy stores prior to the resumption of migration, which can increase an individual's likelihood of survival. Carryover effects from prior habitats may mediate stopover behavior: evidence suggests that low‐condition migrants exhibit longer stopovers and attain more growth during stopover than high‐condition individuals. For Pacific salmon (Oncorhynchus spp.), estuaries may be important stopover habitats during juvenile emigration from natal freshwaters to the ocean. Favorable foraging conditions in high‐productivity estuaries could boost growth, and body size at marine entry is generally positively correlated with survival to adulthood. However, the contribution of the estuarine life stage to individual fitness across the salmon life cycle remains poorly understood. Here, we show that body size at freshwater exit mediates estuary stopover behavior by juvenile coho salmon (O. kisutch) and that estuary growth can increase the probability of marine survival. We performed a multiyear mark–recapture study of juvenile coho salmon in the Koeye River and its estuary, located on the Central Coast of British Columbia. We found that stopover duration decreased with increasing body size: small juveniles spent more than three times longer in the estuary than their larger conspecifics. Mean annual estuary growth rates ranged from 0.58 to 0.95 mm/day, and the smallest juveniles grew by nearly 30% in fork length during estuary stopover. Consequently, each out‐migrating juvenile cohort entered the ocean larger and less variable in body size than it was at freshwater exit. Analysis of returning spawners from two juvenile cohorts (2017, 2018) indicated that larger individuals had higher ocean survival rates. Using a simplistic analytical extrapolation, we demonstrate that estuary stopover could increase marine survival: For median‐sized juveniles, growth achieved in the estuary was associated with 39.8% (34.0%–46.2%) higher survival for the 2017 cohort and 39.1% (31.6%–47.0%) higher survival for the 2018 cohort. These results, from a watershed unaltered by intensive logging or development, suggest that growth achieved during estuary stopover can mitigate freshwater carryover effects and buffer size‐selective survival in a changing ocean. More broadly, these findings highlight the importance of state‐dependent use of stopover habitats in the study of migration ecology and conservation of migratory species.