Vertical distribution, seasonal occurrence and growth of larval and juvenile (<50 mm standard length [SL]) Bathylagus ochotensis were investigated utilizing discrete-depth and monthly oblique-haul samples at a fixed station in Sagami Bay, central Japan. Unlike other midwater
fishes, the larvae did not occur in the upper 200-m layer, remaining in the upper mesopelagic zone (ca. 300–600 m) during both day and night. They exhibited a clear size-depth stratification within the latter depth range, larger individuals occurring deeper. Seasonality in occurrence
was evident; larvae being recorded from October to May, with abundances peaking in January and February. Modal progressions in monthly length-frequency distributions indicated that B. ochotensis metamorphoses at around 18 mm SL, about 3 months from hatching, and attains 34 mm SL after
1 year, at which time all individuals are juveniles. Extrapolation of growth curves, supplemented by observations on adult specimens, suggested that males and females attain sexual maturity after 3 and 4 years (at about 70 and 100 mm SL), respectively.
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