MEPS 575:119-136 (2017)  -  DOI: https://doi.org/10.3354/meps12198

ABSTRACT: The ecology of leptocephali is poorly understood. We investigated their nighttime vertical distributions and assemblage structure in the North Equatorial Current (NEC) region of the western Pacific in October and November 2013. Of the 1372 collected leptocephali of about 66 species, the most abundant taxa were Serrivomeridae, Nemichthyidae, Conger, and Anguillidae that were spawned offshore and Ariosoma, Muraenidae, and Chlopsidae larvae that had been transported offshore. Horizontal towing along the 141°E transect indicated that all species of leptocephali were more abundant at 50–100 m near or above the top of the thermocline than at 150–200 m. Towing in the other areas indicated that Ariosoma, Avocettina, Nemichthys, and Serrivomeridae leptocephali were more abundant in the surface mixed layer at 30–50 m, while Anguilla, Conger, Derichthys, Chlopsidae, Muraenidae, and Ophichthidae leptocephali were more abundant near the top of the thermocline at 70–100 m. Leptocephali were most abundant and diverse in the western 131°E transect, and 3 main assemblage groups of leptocephali were detected based mostly on the distributions of oceanic, anguillid, and congrid taxa. Anguillid and conger eels were spawning in specific areas after offshore migrations, and Avocettina, Nemichthys, and Serrivomeridae had spawned over wide areas. The vertical distribution patterns support the hypothesis that nighttime vertical distributions of leptocephali are influenced by the thermocline. The assemblage structure of the NEC region appears to be a reflection of the taxa spawning offshore and other leptocephali being transported offshore by countercurrents and gyre recirculation.

KEY WORDS: Leptocephali · Vertical distribution · Assemblage structure · North Equatorial Current · Spawning area