The effect of moderate exercise on growth and aggression depending on social rank in groups of Arctic charr ( Salvelinus alpinus L.)

Abstract

Several studies have shown that prolonged moderate exercise improve conditions for the farming of salmonids, by reducing aggressive interactions and resulting in enhanced growth and reduced size variation. The present study focuses on the effect of forced moderate exercise on growth and aggression depending on the individual social rank, compared to non-exercised controls in groups of Arctic charr ( Salvelinus alpinus L.). Social screening was done prior to the trial in each of 24 groups with five charr, where every fish was ranked into one of five social ranks by successive removal of individuals displaying a dominant or aggressive behaviour. Four ranking groups (= 20 fish) were then merged into one experimental group (= 6 groups), each with four tagged individuals of ranks 1–5. After 62 days with or without forced exercise, no significant difference in mean Daily Growth Coefficient (DGC) in the exercised (DGC = 1.25 ± 0.46) and non-exercised (DGC = 1.39 ± 0.47) groups were found. A significant relationship between rank and size at the start remained significant in the non-exercised groups but became insignificant in the exercised groups. It was the highest ranked fish that gained by not being forced to exercise. They had a significantly higher weight gain than the highest ranked fish in the exercised groups and all lower ranked fish independent of treatment. The observed aggression was significantly higher in the non-exercised groups and higher in between than during feeding sessions.