Aquat. Living Resour.
Volume 13, Number 4, July-August 2000Mechanisms and Effects of the Aggregation of Tuna by FADs
|Page(s)||233 - 240|
|Published online||15 July 2000|
Food consumption of tuna in the Equatorial Atlantic ocean: FAD-associated versus unassociated schools
present address: IRD, Dep. HEA, BP 5045, 34032 Montpellier cedex 1, France
2 IRD, Centre de recherches océanologiques, BP V 18 Abidjan, Ivory Coast
3 IOTC, PO Box 1011 Victoria, Mahe, Seychelles
4 IRD, Institut océanographique, 195 rue Saint-Jacques, 75005 Paris, France
Accepted: 3 July 2000
Since 1991, fishing operations on tuna schools associated with drifting Fish Aggregating Devices (FADs) have become widespread in the purse seine fishery in the Gulf of Guinea. In the offshore South Sherbro area (0–5° N, 10–20° W), FAD-associated catches represent about 75 % of the total catch. This FAD fishery exploits concentrations of skipjack mixed with a smaller amount of bigeye and yellowfin tuna of similar size (46 cm), and some large yellowfin. Catches on unassociated tuna schools are mainly composed of large yellowfin in breeding phase and skipjack. Here we studied tuna diet in relation with the aggregation mode (FAD-associated or unassociated tuna schools), species, and size. The stomach contents of around 800 fish were analysed. Numerous empty stomachs were found, especially in fish caught under FADs. Diets were similar for all small-size tuna sharing the same aggregation type. Small tuna mainly feed on Vinciguerria nimbaria (Photichthyidae), a mesopelagic fish of the micronekton, whereas large tuna mainly feed on Scombridae, mixed with Cubiceps pauciradiatus (Nomeidae) when they were caught in unassociated schools. The feeding habits of tuna are discussed with emphasis on the behavior of V. nimbaria. Estimations of the daily ration of similarly sized tuna with the same aggregation mode were very close. The low estimated rations for small, FAD-associated tuna show that logs do not have a trophic function, but rather are a refuge. In contrast, FADs seem to influence the diet of large tuna because of the Scombridae prey that probably is associated to the FAD.
Key words: tuna feeding / daily ration / Fish Aggregating Devices / Vinciguerria nimbaria / purse seine / Atlantic Ocean
© Elsevier, Inra, Ifremer, Cemagref, Ird, Cnrs, 2000
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