Behavioral and gene expression changes in response to social chronic stress in rainbow trout (Oncorhynchus mykiss): characterizing depression-like states in fish

Chronic social stress is a major challenge in aquaculture, as it can compromise growth, health, and welfare in farmed fish. Hierarchical interaction between fish in dense population are difficult to avoid, meaning that subordinate individuals are typically chronically stressed and continually exposed to environmental stressor. Prolonged exposure to stressors can result in a sustained depression-like state (DLS). This thesis investigates how prolonged exposure to social dominance hierarchies affects behaviour and gene expression in juvenile of rainbow trout (Oncorhynchus mykiss), with a focus on the characterization of depression-like states (DLS) and is reversibility. Pairs of fish were subjected to social interaction experiments over several weeks, during which the interaction time was gradually increased. Several behavioural phenotypes were identified: Dominant, Coping and DLS subordinate. The subordinate individuals classified as DLS, were characterized by decreased, reduced feeding motivation, and modified expression of stress and plasticity genes, compared to both dominant and coping subordinate (i.e. subordinates under social stress that did not develop a DLS profile) individuals. In aquaculture, the fish that display symptoms of DLS, represent a welfare and productivity challenge. To examine reversibility, DLS fish were isolated. These individuals showed signs of behavioural and physiological recovery, suggesting that DLS may not be a permanent state, at least in short term.