FRONT INDIVIDUALS LEAD IN SHOALS OF 3-SPINED STICKLEBACKS (GASTEROSTEUS-ACULEATUS) AND JUVENILE ROACH (RUTILUS-RUTILUS)

Fish shoals are usually seen as anonymous leaderless groups in which a ll individuals have the same influence on swimming velocity and direct ion. This hypothesis was tested by investigating swimming directions o f shoals of roach (Rutilus rutilus) and three-spined stickleback (Gast erosteus aculeatus). In roach, the influence of front and rear fish on the shoal's swimming direction was compared by analysing video record ings. Front fish initiated new directions significantly more often and were followed by rear fish. In a Second experiment two shoals of stic klebacks were released from two channels which were positioned at an a ngle relative to each other. The shoals usually appeared with a short time difference at the opening of the channels and then merged. Initia lly the two shoals faced in different directions based on the orientat ion of their respective channel and it was recorded which direction pr evailed after the shoals had merged. The shoal that left the channel f irst, and therefore formed the front part of the merged shoal, clearly dominated the direction. Thus, both experiments gave evidence for fro nt fish having a dominant influence on the direction of the shoal. In the context of sustained position preferences of individual fish, rece ntly observed in roach, this suggests that fish shoals may have leader s over extended time periods.