Vertical distributions, diel and ontogenetic vertical migrations and net avoidance of leptocephali of Anguilla and other common species in the Sargasso Sea

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Abstract
Day- and night-time vertical distributions and their ontogenetic changes in Anguilla leptocephali and other common species of leptocephali were determined and compared during five cruises in the Sargasso Sea using an opening - closing 2-m ring net to sample discrete depth strata between 0 m and 350 m deep. No difference in vertical distribution was ever found between Anguilla rostrata (American eel) and A. anguilla (European eel). Anguilla leptocephali Anguilla ≥5 mm did perform a diel vertical migration. Anguilla of the length range 5.0–19.9 mm mostly occurred between 100 m and 150 m by day and between 50 m and 100 m by night. Anguilla ≥20 mm were found deeper than Anguilla Nemichthys scolopaceus N. scolopaceus exhibited a diel vertical migration, as most were caught below 100 m by day and above 100 m by night. There was a distinct vertical migration in Derichthys serpenrinus . Larvae of this species mostly occurred between 125 m and 150 m by day and between 30 m and 70 m by night. The other common species of leptocephali ( Serrivomer beani, S. breviden-tatus, Ariosoma balearicum, Conger oceanicus, Avocettina infans, Nessorhamphus ingolfianus, Anarchias yoshiae, Gymnothorax sp ., and Kaupichthys hyoproroides ) were mostly found in the upper 100 m of the ocean at night. The ascent of leptocephali seemed to occur rapidly, mostly in the hour following sunset, and the descent to occur more gradually. However, interpretation of these data was difficult because of net avoidance. A high index of visual avoidance by day for most leptocephali, calculated from the ring net series, indicated that most leptocephali were efficient at seeing and escaping the net during daytime, but visual avoidance by day was not important in small Anguilla .

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