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Dominance hierarchies in fish are established through conflicts and lead to significant differences in stress physiology, behaviour, and immune function between dominant and subordinate individuals. This review explores the relationship between social stress within these hierarchies, individual variations in stress responses and cognitive bias towards stressful situations, and the impact of conflicts on future performance and interactions between stress and the immune system. Fish express divergent stress coping styles (proactive and reactive) that differ in the reactivity of the stress axes ending with the release of the major stress hormones (adrenaline and cortisol). Social stressors, like conflicts, are perceived differently by proactive and reactive fish, leading to varying levels of stress hormone release. The stress hormones interact with the immune system, changing individuals' ability to fight off pathogens. Pro-inflammatory cytokines highly activated in reactive individuals under stress can provide feedback to the monoaminergic system in the brain, resulting in depression-like, anxiety-like, or "sickness" behaviour. The review also discusses strategies for reducing social stress in fish and enhancing their overall health in aquaculture, while emphasising the importance of considering these factors in research settings to prevent data bias.

Individuals with divergent personality traits corresponding to stress coping styles have been suggested to differ in behavioural and neural plasticity. We used a model of social defeat stress to assess the coping ability of wild zebrafish selectively bred for boldness/shyness. Behavioural tests were applied to assess parameters such as boldness/exploration, aggressiveness, and displacement behaviour. Gene expression changes in the brain were assessed via RNA sequencing. The main results show a strong effect of shyness and boldness phenotype on behaviour and the brain transcriptome. Fish of the shy line displayed significant behavioural differences, while the number of differentially-expressed genes remained low. In contrast, fish of the bold line exhibited a small effect on behaviour and pronounced changes in brain gene expression. This study highlights the importance of boldness phenotype and its influence on the response to social challenges at the behavioural and transcriptomic levels.

Personality traits and acquired experience affect the capacity of an individual to cope with environmental and social changes. Behavioural adaptation and physiological alterations are important to prepare the body for these potential challenges. Whether inherited traits or acquired social rank (reflecting stress levels) are more important and how different personality-social rank combinations affect an individual's health is not well understood. One important aspect of health status is the function of biological barriers, as they represent the first line of defence of an organism. In the current study, we used a model of social defeat stress applied to a bold and a shy line of zebrafish. The Fulton's condition factor was determined, and gene expression analysis was performed on skin and intestines. The differences between lines explained a major part of the transcriptional changes observed as compared to differences in social rank. Additionally, shy fish that experienced repeated social defeat presented a poor body condition, accompanied by changes in gene expression suggesting inflammation in the gut. In the skin, shy fish showed a transcriptional enrichment of pathways related to cell division as well as increased expression of the stress response-associated gene crh2r. Together, these results complement our previous work and show that shy loser fish experience important changes not only in behaviour but also in their biological barriers, potentially putting their overall health at higher risk.