RANKING OF FEEDING POSITIONS BY DRIFT-FEEDING ARCTIC GRAYLING (THYMALLUS-ARCTICUS) IN DOMINANCE HIERARCHIES

Field experiments in the pools of a mountain stream demonstrate that Arctic grayling (Thymallus arcticus) rank feeding positions according to desirability and that competition sorts fish so that the dominance rank of each individual matches the rank desirability of its position. Groups containing the same number of fish always occupied the same set of positions, and positions were added (in reverse order of desirability) as group size was increased; these results show that fish ranked positions. There was an almost perfect correlation between the dominance rank (measured as fish length) of each fish and the rank desirability of its position, suggesting that competition sorts fish among positions. This conclusion was strengthened by the results of sequential removal experiments in which the dominant fish was removed at the end of each day. After each removal the remaining fish almost always moved into the positions previously occupied by fish immediately above them in the dominance hierarchy.