Effects of environmental enrichment on forebrain neural plasticity and survival success of stocked Atlantic salmon

Abstract

Fish reared for stocking programs are severely stimulus-deprived compared to their wild conspecifics raised under natural conditions. This leads to reduced behavioural plasticity and low post-release survival of stocked fish. Environmental enrichment can have positive effects on important life-skills, such as predator avoidance and foraging behaviour, but the neural mechanisms underpinning these behavioural changes are still largely unknown. In this study, juvenile Atlantic salmon (Salmo salar) were reared in an enriched hatchery environment for seven weeks, after which neurobiological characteristics and post-release survival were compared to fish reared under normal hatchery conditions. Using in situ hybridisation and qPCR, we quantified the expression of brain-derived neurotrophic factor (bdnf) and the neural activity marker cfos in telencephalic subregions associated with relational memory, emotional learning, and stress reactivity. Aside from lower expression of bdnf in the Dlv (a region associated with relational memory) of enriched salmon, we observed no other significant effects of enrichment in the studied regions. Exposure to an enriched environment increased post-release survival during a five-month residence in a natural river by 51%. Thus, we demonstrate that environmental enrichment can improve stocking success of Atlantic salmon parr and that EE is associated with changes in bdnf expression in the fish's hippocampus-equivalent structure.