Fish shoals are usually seen as anonymous leaderless groups in which all individuals have the same influence on swimming velocity and direction. This hypothesis was tested by investigating swimming directions of shoals of roach (Rutilus rutilus) and three-spined stickleback (Gasterosteus aculeatus). In roach, the influence of front and rear fish on the shoal's swimming direction was compared by analysing video recordings. Front fish initiated new directions significantly more often and were followed by rear fish. In a second experiment two shoals of sticklebacks were released from two channels which were positioned at an angle relative to each other. The shoals usually appeared with a short time difference at the opening of the channels and then merged. Initially the two shoals faced in different directions based on the orientation of their respective channel and it was recorded which direction prevailed after the shoals had merged. The shoal that left the channel first, and therefore formed the front part of the merged shoal, clearly dominated the direction. Thus, both experiments gave evidence for front fish having a dominant influence on the direction of the shoal. In the context of sustained position preferences of individual fish, recently observed in roach, this suggests that fish shoals may have leaders over extended time periods.
We modified Hamilton's (1971) selfish herd model by introducing directional movement to the prey groups and the predators. The consequences of this modification with regards to differential predation risks are compared to Hamilton's original model (using stationary prey groups) and tested against empirical data. In model 1, we replicated Hamilton's original predator-prey system. In models 2 and 3, prey groups were mobile and predators were mobile (model 2) or stationary (model 3). Our results indicate that additional to the positive risk gradient from centre to periphery predicted by Hamilton's model for stationary groups, there might be another positive risk gradient from the rear to the front part in moving groups. Furthermore, models 2 and 3 suggest that moving groups should generally exhibit an elongated shape (longer than wide along the axis of locomotion) if risk minimisation is the only factor concerned. Also smaller inter-individual distances are predicted for front individuals than individuals elsewhere in the group. Empirical data based on the three-dimensional structure of fish shoals (using roach, Rutilus rutilus) were consistent with the above two predictions. A second experiment which involved lake chub, Semotilus atromaculatus, as prey and rock bass, Ambloplites rupestris, as predators, provided direct support for the hypothesis that individuals in front positions of groups incurred a significantly higher predation risk than fish in rear positions. Finally, we discuss the differential risks of different group positions in the context of potential foraging gains which provides the basis for a dynamic model of position preferences in group-living animals.