Environmental conditions influence exploration, antipredation behavior, and fin condition in juvenile Chinook salmon (Oncorhynchus tshawytscha)

Differences in environmental conditions experienced throughout development are known to produce differences in phenotypic traits. Hatchery and wild fish often express different phenotypic traits driven, at least in part, by differences in the early rearing environment. These differences can influence their ability to adapt and survive in the wild. In this study, we compared juvenile Chinook salmon (Oncorhynchus tshawytscha) that were reared at high and low density, with and without complex structure, and used a psychological test to evaluate impacts on behavior, including motivation, exploration, and what we consider to be an antipredation response. Juvenile Chinook salmon are generally motivated to be near conspecifics. In the test, fish were presented with one of three stimuli (conspecifics, structure, or nothing) across an aversive area from their starting location (start box). Fish reared in low density spent more time in the start box and had a longer latency to leave the start box (for those that left without bolting) compared to those reared in high density, regardless of the stimulus presented. However, there was no impact of structure on these behaviors, although those reared with structure were more likely to either bolt out of or stay in the start box. Rearing treatment also influenced caudal fin condition such that fish reared in low density had larger and more pointed fins than fish in high density and fish reared with structure had larger and more symmetrical fins than those reared without structure. We speculate that reduced density and rearing with structure led to fish with enhanced antipredation responses which may confer adaptive advantages if released in a novel environment and possibly improve their survival upon release in the wild.