Effects of early rearing environment and breeding strategy on social interactions and the hormonal response to stressors in juvenile Chinook salmon

To determine whether early rearing environment and parental breeding strategy affect the social behaviour and the endocrine response to stressors in juvenile Chinook salmon (Oncorhynchus tshawytscha), offspring (1–2 g) from traditional hatchery breeding or parental mate choice breeding were reared in a hatchery setting or in seminatural channels. Once ∼30 g, 9-month-old hatchery and mate choice fish from both rearing environments were exposed to one of four treatments: (i) sampled, (ii) air-exposed (AE) for 60 s and sampled 1 h later, (iii) sampled after 5 days of continuous dyadic social interaction (SI), or (iv) AE and allowed to interact for 5 days (AE/SI). In the hatchery environment, while hatchery fish were dominant in 70% and 80% of the dyadic trials in the SI and AE/SI treatments, respectively, plasma cortisol, growth hormone, and insulin-like growth factor I levels did not differ between hatchery and mate choice fish. In contrast, when reared in a seminatural environment, mate choice fish were dominant in 70% of the dyadic trials in the SI and AE/SI treatments, and clear differences in plasma hormone levels emerged between hatchery and mate choice fish. Therefore, while we found no evidence that breeding strategy affects social status, familiarity with the early rearing environment (i.e., from emergence until 1–2 g) enhanced the competitive ability of juvenile Chinook salmon during dyadic interactions. Early rearing environment also affected the endocrine responses to stressors, and freshwater seminatural channel environments were associated with elevated hormonal responsiveness.