No one knew why and it didn't matter, that behavioral mechanism was just used to catch more fish in the Mediterranean. Today, artisanal and industrial tuna fisheries exploit this “aggregating phenomenon” in much the same way. Over the last thirty years, seine fishing in particular has developed rapidly through the use of massive floating objects, natural at first, then more recently fish aggregation devices (FADs) remotely monitored using electronic beacons.
These floating objects help enable 40 % of worldwide tropical tuna catches today.
Despite the prevalence of floating objects in fishing around the world, science has not known much more than the Romans about the behavioral mechanisms involved, due to the difficulty of making observations and doing experiments at sea. Institut de Recherche pour le Développement (IRD) partners recently conducted initial experiments around Hawaii in the Pacific. They fitted over 70 albacore with acoustic transmitters and showed that their stay time under the fish aggregation devices is unexpectedly highly varied: it is either very short, less than three days, or very long, up to 23 days on average.
Under a natural wreck. Credit: IRD-Ifremer/Fadio/M. Taquet
The time spent under an object therefore depends on the environment, or on the presence of fellow fish, without which the tuna would stay for a varied time under each device.
Noted social behavior in this aggregating phenomenon
Other experiments were conducted offshore - namely “binary choices” which had previously only conducted in the laboratory on insects or small fish. They anchored pairs of identical FADs only 5 yards apart in the Seychelles in the Indian Ocean. Using sonar-fitted buoys, they then compared the quantity of tuna gathered under each device. Result: one of the floating objects attracted more individuals than the other.
The proximity of the two FADs means that external stimuli, such as the zone’s richness, can be considered as similar under each FAD, highlighting social behavior on the part of the tuna. Attraction to the floating object alone would have generated identical distributions of tuna between the two objects.
This work on the behavioral mechanisms of tuna is being used to develop models to assess the impact of fish aggregation devices on tuna migrations and biology, still poorly understood. In particular, it can be used to establish whether FADs, used in their thousands by fishermen, are “ecological traps”, able to attract fish towards unfavorable areas.
ROBERT M., DAGORN LAURENT, LOPEZ J., MORENO G., DENEUBOURG J. L. Does social behavior influence the dynamics of aggregations formed by tropical tunas around floating objects ? An experimental approach. In: J. Exp. Mar. Biol. Ecol. Journal of Experimental Marine Biology and Ecology, 2013, 440, 238-243.
ROBERT M., DAGORN LAURENT, FILMALTER J. D., DENEUBOURG J.-L., ITANO D., HOLLAND K. Intra-individual behavioral variability displayed by tuna at fish aggregating devices (FADs). Marine Ecology Progress Series, 2013, in press.