Consistent individual competitive ability in rainbow trout as a proxy for coping style and its lack of correlation with cortisol responsiveness upon acute stress

For a given fish species, individuals are different in their ability to cope with stressors; each individual has its own set of physiological and behavioral responses to stress (stress-coping style). This individual diversity is of importance when considering the welfare of fish reared in aquaculture facilities.

In this study with rainbow trout (Oncorhynchus mykiss) we investigated the link between the ability to compete for food of each individual (used as a proxy of dominance behavior/proactive stress-coping style) and its ability to cope with stress; we hypothesized that fish that are better competitors would be more robust against common aquaculture stressors.

We screened 680 rainbow trout individuals for competition ability. This was done by submitting groups of 20 individuals to a 1-week competition trial where they were kept at low stocking density and were provided a restricted amount of food. A 15% of the screened fish were selected as “winners” and another 15% were selected as “losers”, based on growth rates during the competition trials.

Fish were re-tested in a second competition trial after several weeks, to assess for consistency of competitive ability. Winner and loser fish were individually exposed to confinement and their neuroendocrine stress response was evaluated (serotonergic activity in telencephalon and brain stem, plasma levels of cortisol, glucose and lactate).

Furthermore, behavioral responses to confinement and net restraining tests were also investigated. The results showed good temporal consistency of competitive ability in the lapse of time of the experiments. Besides, competitive ability showed a positive association to fish activity during the net restraining tests.

However, plasma stress marker data showed a lack of relevant differences between the acute stress responses of winner and loser fish, adding up to the body of evidence suggesting that stress responsiveness might not be consistently linked to SCS in vertebrates. This, together with the inability of winner fish to outperform loser fish in usual stocking density conditions, suggests that there is no clear welfare or performance benefits in selecting fish of a specific coping style for fish farming, at least in the domesticated trout population used in the current study.