Schooling properties of an obligate and a facultative fish species

Abstract : Changes in attraction and repulsion indicators, depending on the species and the group size, were explored under controlled conditions. Two species, displaying different schooling behaviours in the wild were observed: the bigeye scad Selar crumenophthalmus and the barred flagtail Kuhlia mugil. In the bigeye scad, the polarity and speed were high and stable, and the nearest neighbour distance (D-NN) decreased when the group size increased. In contrast, for the barred flagtail, polarity and speed decreased according to the group size, inducing a loss of cohesion and leading to a disorganized school. The D-NN mean was stable whatever the group size and relatively high. This experiment indicated that the ability to polarize is first a species-specific trait, rather than a property emerging from the group and led by the circumstances.