Nighttime vertical distribution and regional species composition of eel larvae in the western Sargasso Sea

• Vertical and horizontal distributions of leptocephalus larvae were investigated.
• >47 species collected by MOC-10 at 5 western Sargasso Sea stations.
• Differences in species distributions and vertical distributions were observed.
• Thermocline depth may influence nighttime vertical distributions.

Eel larvae, called leptocephali, are widespread throughout the surface layer of tropical and subtropical oceans, but their ecology is poorly understood and few studies have examined their vertical distributions. The species composition and vertical distribution of leptocephali was studied in the western Sargasso Sea using a 10 m2 mouth-opening MOCNESS-10 trawl system with multiple nets that sampled 4 discrete depth ranges at 5 night stations extending from the northern Florida Current (FC) to near the northern Bahamas in July–August 1993. Nets mostly fished in the upper 200 m (max. depth 600 m), and collected 469 leptocephali of >47 species from 11 families (size range: 20–260 mm). Anguilla leptocephali (32–53 mm) were caught at each station at depths from 0–30 m to 40–70 m, with only one individual collected at 90–120 m. Some marine eel taxa had different vertical distributions and their species composition varied geographically. The congrid Rhynchoconger flavus and nettastomatid Hoplunnis macrura were only abundant along the western side of the FC in the north at 50–100 m depths. Ariosoma selenops leptocephali and a richer assemblage of species were present near the eastern edge of the FC, with the majority caught at 0–50 m. Farther south in the recirculation region east of the FC, >33 species were collected, with Anguilla rostrata, congrids, chlopsids, moringuids, ophichthids, and muraenids being most abundant at 0–30 m, but nemichthyids and derichthyids were most abundant at 30–60 m. Similar depth ranges were observed at the southern stations, and no leptocephali were collected in the 400–600 m depth range. These data indicate there is variability in the fine-scale vertical distributions of leptocephali within the upper 100 m at night and that few of these larvae are present in deeper layers offshore.