Salmon choose to 'exercise' and show signs associated with less stress

Is exercise something that salmon simply tolerate, or can they also actively seek it out?

A recent study provides an interesting answer: when juvenile Atlantic salmon were given the choice, they spent a significant portion of their time swimming against the current.

The study, developed by scientists from Norway and Sweden, sought to address a little-explored question in fish, which relates to whether "voluntary exercise" can generate responses associated with motivation and well-being, as occurs in mammals.

The difference between this study and others, according to the authors, is that the fish were not forced to swim in currents to simulate exercise, but were given the option to choose.

To do this, the experts used an experimental system where groups of five fish could move freely between an area with current and another of still water. In a control group, however, the entire area remained without currents.

The power of decision

Fish with access to water currents progressively used the flow zone more during the trial, especially when currents reached the highest speeds, between 1.5 and 1.8 body lengths per second.

Interestingly, the salmon did not enter the area with current constantly. They did so less frequently than the fish in the control group entered the same area (without current), but they remained for much longer periods.

“Schools that chose to exercise entered the current zone less frequently than the controls, but in the last 20-minute interval, their longest episode in the current occupied an average of 96% of the interval. Nonetheless, the fish that chose to exercise continued to use the calm zone at all times, spending 8.8% of the recorded time there, compared to 27.6% in the controls,” the scientists explained.

Flexible use

The above suggests that the behaviour did not simply correspond to an absolute preference for flow, but to a flexible use of both conditions.

Additionally, the choice to swim in currents was accompanied by physiological differences. The groups that had access to the current showed, on average, 28% less plasma cortisol, a hormone widely used as an indicator of stress response in fish. Furthermore, they exhibited increased dopaminergic and serotonergic signalling in the dorsomedial pallium, a brain region linked to affective processing.

For aquaculture, the finding raises a specific question: could the design of the farming environment offer fish opportunities for exercise that they themselves regulate?

The researchers propose to study in greater depth the combination of flowing zones and calm areas as a possible tool for environmental enrichment and welfare in farmed salmon.

“Our results suggest that the implication is not that flow should be imposed, but rather that heterogeneity within the flow should be provided. Ideally, areas of higher speed should be adjacent to calm areas that allow for rest, so that what is offered is the ability to make choices about swimming rather than just swimming effort. This experiment demonstrates self-selected exercise in a non-mammalian species,” the authors concluded.

Read the full study titled "Behavioural and neuroendocrine correlates of swimming exercise choice in juvenile Atlantic salmon" here.