Reconstructing water temperatures and depths experienced during the newly hatched larval stage of the tropical eel Anguilla marmorata using otolith oxygen isotope analysis

Abstract The extensive oceanic life phase of anguillid eels, which occurs far from their freshwater habitats, presents significant challenges in investigating their early life ecology. This is especially true for tropical eel species, which remain largely understudied. In this study we investigated temperature exposure and vertical depth distribution experienced during the newly hatched larval stage of the tropical eel Anguilla marmorata glass eels in the western North Pacific, analyzing stable oxygen isotope ratios (δ 18 O) in the otolith core of 78 glass eels using secondary ion mass spectrometry. Otolith microstructural analysis indicated year-round spawning, with glass eels hatched in the fall showing a significantly longer oceanic phase. The otolith core δ 18 O values ranged from −3.3‰ to −1.8‰ (VPDB), corresponding to water temperatures of 19.5°C to 29.1°C and depths ranging from 51 to 213 m. No significant differences in experienced temperature or depth were observed among larvae hatched in different seasons, suggesting consistent environmental conditions encountered by newly hatched A. marmorata larvae throughout the year. The estimated depth range of A. marmorata larval occurrence was broader than that of the Japanese eel Anguilla japonica , despite their overlapping ranges. The extensive depth range estimated to have been experienced during the newly hatched larval stage of A. marmorata may represent dispersal strategies that are aimed at reducing interspecies competition for food and to maximize their survival during the most vulnerable early life history stage.