Does social behavior influence the dynamics of aggregations formed by tropical tunas around floating objects? An experimental approach

Tropical tunas associate with objects floating at the surface of the ocean, a behavior widely exploited by fish- ers. However, the respective roles played by environmental variables and behavioral processes (e.g., social behavior) in the formation of these aggregations remain elusive. To investigate the role of social behavior in the dynamics of such aggregations, we used the binary choice approach. The experimental design com- prised two close and identical anchored fish aggregating devices (FADs) equipped with an echo sounder buoy to monitor the aggregated biomass of tuna under each device. Analysis of the results entailed character- izing whether the aggregated biomass is distributed asymmetrically (indicative of social behavior playing a role in the dynamics) or symmetrically between the two close and identical FADs, and comparing the results with theoretical distributions based on different definitions of basic units (individual fish or small schools). The results suggest that social interactions underlie aggregation processes, which represents a major advance in our understanding of these aggregations, a priority for science-based fishery management. While recogniz- ing the logistical and technical constraints, we encourage the development of experimental studies (e.g., in which animals are presented with controlled situations) to enhance our understanding of the behavior of large pelagic fish.