Exploitation of small tunas by a purse-seine fishery with fish aggregating devices and their feeding ecology in an eastern tropical Atlantic ecosystem

We investigated the effects of a purse-seine fishery with drifting fish aggregating devices (FADs) in the South Sherbo area of the Equatorial Atlantic, located between 0-5N and 10-20W. There had been no surface fishing activity in the area until 1975. Since 1991, fishing operations on schools of tuna associated with FADs has become widespread and this offshore area has developed into a major fishing zone. Exploi- tation rates are high between November and January. The fishery exploits multispecies concentrations of skipjack (71%), bigeye (15%), and yellowfin (14%) tunas of similar size (mode: 46-cm forklength). The use of FADs increased the vulnerability of small tunas and induced changes in fishing patterns. The mean individual weight of skipjack caught has decreased since 1991, due either to overfishing or to a growth change. Data from scientific observers were used to estimate discards and by-catches generated by FAD fishing during 1998. Discards of tunas (including frigate and little tunas) represented 7.6% of the total catch. Other by-catch (dominated by wahoo, billfish, triggerfish, sharks, barracudas, and dolphinfish) represented 2.3%, including 0.4% discarded at sea. Stomach content analysis showed that a mesopelagic species, Vinciguerria nimbaria (Photichthyidae), which during daylight concentrated in the upper layers in dense schools, was the main prey of all small tunas. The South Sherbro area appears to have exceptional environmental conditions. It is suggested that the FAD fishery may have wide-ranging effects on the migration of tunas in general and on the productivity of the skipjack population in particular.