Shoal choice in juvenile angelfish (Pterophyllum scalare): effects of social status and activity

Abstract

Previous experiments have shown that angelfish are capable of discriminating between shoals composed of conspecifics of different social status and that familiarity mediates the decision. In the present work, the shoaling behaviour of dominant and subordinate angelfish, Pterophyllum scalare, was again examined and the role that potential behavioural differences between the stimulus shoals may play in the shoaling decisions was investigated. In binary shoal choice tests, the stimulus shoals were composed of dominant and subordinate conspecifics from the same dominance hierarchy as the test fish but, in contrast with most studies of this type, the stimulus shoals were placed in tanks that allowed for the full expression of behavioural patterns. Test fish, regardless of social status, showed an association preference for a shoal of dominant over a shoal of subordinate conspecifics. The tendency was significant in the second observation period, which took place about 2 hr and 30 min after the first one, when the swimming activity of the stimulus shoals of dominants was significantly greater than that of subordinates and the shoaling preference for dominants was significantly related to activity differences between the shoals. The results contrast with those previously found in this species when test fish preferred to aggregate with shoals of familiar subordinate conspecifics placed in small display compartments and indicate that angelfish take shoal activity into account when making a shoaling decision. Thus, depending on the situation, shoal activity may take priority over other attributes of the shoals. The preferential association is considered in the context of the potential benefits to the fish when joining a group of active fish in our experimental situation.