July 29, 2013
Mysterious Links: Tuna And Floating Objects
A boon for fishing
More than 2 000 years ago, Roman fishermen already used the natural propensity of some species of fish to gather under floating objects, to enhance their catches in the Mediterranean. Today, numerous industrial and artisanal tuna fisheries around the world exploit this "aggregating phenomenon". 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 (abbreviated to FAD) remotely monitored using electronic beacons. Today, these floating objects enable 40 % of worldwide tropical tuna catches.
Highly varied residence times
Despite considerable use of floating objects around the world, until now scientists had hardly studied the behavioral mechanisms involved, due to the difficulty of making observations and doing experiments at sea. To make up for this lack of knowledge, IRD scientists and their partners conducted initial experiments around Hawaii in the Pacific. They fitted over 70 albacore with acoustic transmitters and showed that their stay time under the FADs is unexpectedly highly varied: it is either very short, less than three days, or very long, up to 23 days on average. 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
To back up these initial results, other experiments were conducted offshore, called "binary choices", previously only conducted in the laboratory on insects or small fish. To do so, they anchored pairs of identical FADs only 5 km apart, around 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.
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