ABSTRACT

Acknowledgments.................................................................................................................................. 196 References .............................................................................................................................................. 197

The tuna resources in the Eastern Central Atlantic have been the object of both intensive Þshery for over 30 years and numerous studies conducted under the coordination of the International Commission for the Conservation of Atlantic Tunas (ICCAT) (Fonteneau and Marcille, 1993). The skipjack tuna (

Katsuwonus pelamis

, henceforth referred as “skipjack”) supports an important commercial Þshery across the Eastern Atlantic from the Gulf of Guinea to the southwestern Irish coast (ICCAT, 1986). Tag recovery studies have indicated that skipjack migration routes lie from the southeast toward the north and northwest Atlantic (Ovchinnikov et al., 1988) and catches on the highly migratory tuna stocks are due to multigear Þsheries (Fonteneau, 1991). Also, both the spatiotemporal distribution and abundance of skipjack tuna have been related to causes such as environmental requirements and feeding (Ramos et al., 1991), upper ocean dynamics (Ramos and Sangrá, 1992), hydro climatic factors (Pagavino and Gaertner, 1994), prey abundance (Roger, 1994a, b; Roger and Marchal, 1994), thermal habitat (Boehlert and Mundy, 1994), schooling behavior (Bayliff, 1988; Hilborn, 1991), mesoscale frontal ocean and upwelling dynamics (Fiedler and Bernard, 1987; Ramos et al., 1991), as well as several other aspects beyond the scope of the present chapter. Skipjack tuna appears to be able to adapt the feeding strategy to environmental conditions preying upon what it encounters (Roger, 1994a, b) and the 18°C isotherm and 3 ml oxygen per liter isoline are considered lower limiting factors (Piton and Roy, 1983). The exploitation rate on

most tuna stocks has been constantly increasing and assessments have been inefÞcient in estimating the real maximum sustainable yield of those stocks (Fonteneau, 1997). In the Atlantic, tuna catches were suggested to be both underestimated and misreported (Wise, 1985) and, despite the high level of Þshing effort, recruitment overÞshing has never been suggested for skipjack (Fonteneau, 1987).