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

Abstract

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 0-5"N and 10-2o"W. 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. Exploitation 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 0.45/0 discarded at sea. Stomach content analysis showed that a mesopelagic species, Vinciguerria ninzbaria (Photichthyidae), which during daylight concentrated in the upper layers in dense schools, was the main prey of all small tunas. The South Sherbro FAD fishery may have wide-ranging effects on the migration of tunas in general and area appears to have exceptional environmental conditions. It is suggested that the on the productivity of the skipjack population in particular.

Effects of time-area closure on tropical tuna purse-seine fleet dynamics through some fishery indicators

“…The Picolo area is known to support large concentrations of juvenile tunas likely due to oceanographic-specific conditions (Evans et al 1981;Prince et al 2010) and to the abundance of mesopelagic fishes, such as Vinciguerria nimbaria, which is one of the favorite preys of juvenile tunas in this region (Menard et al 2000).…”

Age and growth of three coastal-pelagic tunas (Actinopterygii: Perciformes: Scombridae) in the Florida Straits, USA: blackfin tuna, Thunnus atlanticus, little tunny, Euthynnus alletteratus, and skipjack tuna, Katsuwonus pelamis

“…at 5 kg or less (Menard et al 2000). Small tunas are seasonally important to artisanal and recreational fisheries in many areas around the Atlantic Ocean.…”

Reconstruction of spatiotemporal capture data by means of orthogonal functions: the case of skipjack tuna (<i>Katsuwonus pelamis</i>) in the central-east Atlantic

“…Moreover, in the Gulf of Guinea there are complex process of reproduction, recruitment and behavioural interaction with the environment and fishing strategy that also could increase the variability of skipjack captures Pallares-Soubrier 1996, Solari et al 2003). Ménard et al (2000) 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. The behaviour of the scatter plot of the relationship between reconstructed and observed data suggests that for a high percentage of missing data, over 40%, the fit became worse, probably because the central limit theorem is not satisfied.…”