Schooling properties of an obligate and a facultative fish species

Schooling fish species are conventionally subdivided into obligate and facultative species, depending on the proportion of the time spent in structured schools, but little is known about the factors affecting the group structure in the absence of external structuring stimuli such as food sources, risk of predators or water flow. 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 (Bloch) and the barred flagtail Kuhlia mugil (Forster). In the bigeye scad, the Polarity and Speed were high and stable, and the Nearest Neighbour Distance (NND) 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 NND 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.