Early experience and reproductive morph both affect brain morphology in adult male Chinook salmon (Oncorhynchus tshawytscha)

It is clear that brain size and structure can be greatly influenced by the environment, and in fish, factors such as habitat complexity, rearing environment, and reproductive status have been shown to affect brain morphology and behaviour, but it is unclear how long these effects last. The objective of the current study was to investigate variability in overall brain size and particular brain regions of male Chinook salmon (Oncorhynchus tshawytscha) through the evaluation of potential driving forces — environment and reproductive morph. By comparing fish from different rearing environments and different male reproductive morphs (hooknose versus jack), the current research assessed the influence of each of these factors on overall brain size and on select brain regions. Male hooknose salmon had relatively smaller brains, once corrected for body size, than male jack salmon, suggesting possible trade-offs between somatic and brain development. Fish reared in hatchery environments but released into the wild as presmolts still had relatively larger brains than their wild counterparts, despite sharing wild habitats for over 3 years, suggesting persistent effects of hatchery rearing. Taken together, these results show that the difference in reproductive morphs can substantially impact brain morphology and that short-term environmental influences can have persistent effects throughout ontogeny