First field-based experiment supporting the meeting point hypothesis for schooling in pelagic fish

Abstract

Aggregations of fish around fish aggregation devices (FADs) have been widely described in the literature; most commercial catches of tuna by surface fisheries are performed around FADs, taking advantage of this behaviour. The meeting point hypothesis (MPH) suggests that fish could make use of FADs to increase the chance of encounters between conspecifics, helping individuals to form larger schools. To attempt a validation of the MPH, we performed an experiment in the field to test the following predictions: (1) fish spend more time at FADs than at any other random points and therefore aggregate around FADs, and (2) fish arrive at FADs as isolated individuals or in small groups and leave them in larger groups. Our investigation involved acoustic telemetry techniques commonly used to observe fish at FADs. The study was carried out on a small pelagic fish species, the bigeye scad, Selar crumenophthalmus, in Saint-Paul's Bay (Reunion Island). Our results validated our two predictions: FADs acted as retention points, increasing the encounter rate of fish and enhancing schooling behaviour, thereby supporting the meeting point hypothesis. FADs could be beneficial to the fitness of the associated fish, promote increased school size and hence confer the advantages of being in a larger group. The impact of the deployment of large number of FADs in some ocean regions is reinterpreted in light of our results.