Abstract
Exercise promotes cognitive function and stress resilience in both fishes and mammals. In mammals, these effects are linked to reward mechanisms, particularly when exercise is voluntary. Whether exercise engages comparable mechanisms in fishes remains unclear. This is partly due to salmonid research relying almost entirely on forced swimming, which removes the behavioural agency associated with the enhanced effects of voluntary exercise in mammals. We tested whether juvenile Atlantic salmon would choose to swim against a current when a calm zone (i.e. no current) alternative remained available, and whether doing so was associated with changes in stress physiology and monoaminergic signalling. Schools of five fish were tested once, without acclimation, in an arena offering either a body-length-scaled velocity gradient adjoining a calm zone (exercise choice) or calm water only throughout the arena (control). Exercise-choice schools used the flow zone more than controls used the equivalent space, and increasingly so across the trial. Notably, this escalation in the use of the flow zone was confined to the highest velocities. Moreover, exercise-choice schools entered the flow zone less often than controls, yet by the final 20-min interval their longest single bout in the flow took up an average of 96% of the interval. The exercise-choice fish nonetheless continued to use the calm zone throughout, spending 8.8% of tracked time there against 27.6% in controls. Schools with access to flow were also more cohesive, and the fish had, on average, 28% lower plasma cortisol as well as higher dopaminergic and serotonergic signalling in the dorsomedial pallium, a region implicated in affective processing. This experiment demonstrates self-selected exercise in a non-mammalian species, indicating that joint provision of flowing and calm water, enabling agency over swimming duration, warrants further investigation as a welfare intervention in captive salmon.