Onboard rearing attempts for the Japanese eel leptocephali using POM-enriched water collected in the Western North Pacific

Hatchery produced leptocephalus larvae of the Japanese eel (Anguilla japonica) were reared on a research vessel and fed natural particulate organic matter (POM) collected in the Western North Pacific. A small net (36 cm diameter, 60 µm mesh) was vertically towed from 200 m depth to surface in 2013, and POM-enriched water (POMEW) filtered through 350 µm mesh was fed to the leptocephali for 11 days. Although the swollen gut corroborated the active ingestion of POM by the leptocephali, low survival rate and heavily melanized gut followed by necrosis in the mid-hindgut region of the leptocephali were observed. A large net (1.14 m diameter, 30 µm mesh) was used in 2014 and 2015, which was horizontally drifted subsurface (100–175 m). POMEWs filtered through 53 or 25 µm meshes were fed to the leptocephali for 5–18 days. Neither melanized gut nor necrosis occurred, but considerably low survival rate and little growth comparable with that in a starved condition were observed. No apparent shift in stable carbon and nitrogen isotope ratios was observed in the reared leptocephali. These indicated that the ingested POMs were not assimilated by the leptocephali and suggested that smaller particles may be important for the leptocephali.