Species assemblages of leptocephali in the Subtropical Convergence Zone of the Sargasso Sea

Abstract

Species assemblages of leptocephali are described in relation to density in the Subtropical Convergence Zone (STCZ) of the Sargasso Sea. Seven transects across fronts were made during four cruises in the late winter or early spring of 1983, 1985 and 1989. About 50 species from 13 families of eels were collected at 66 stations, but fewer than 10 species were abundant in all transects. Four oceanic species, two Anguilla species and Conger oceanicus appeared to be spawning in the STCZ. Leptocephali of most species whose adults inhabit the continental shelf were consistently large in size and were more abundant at or south of fronts and in the western transects. Leptocephali of the two most common oceanic species, Nemichthys scolopaceus and Serrivomer beanii, and the most common shelf species, Ariosoma baleaticum, were also more abundant in the western transects, but were abundant at some stations on both sides of fronts. Discontinuities in the assemblages of Anguilla and most shelf species occurred at the location of fronts that formed at the northernmost extent of southern Sargasso Sea surface water (defined as sigma-t < 25.6 kg m-j). These species were rare or absent in mixed convergence zone water (defined as sigma-t 25.2-25.6) north of the fronts. Cluster analysis and ordination of assemblages at 31 night stations reflected the greater species richness and abundance in the west and in the southern water mass. Patterns of assemblage structure within and among transects suggest that convergence of surface water toward fronts in the STCZ may concentrate leptocephali close to fronts and that frontal jets may transport leptocephali eastward.