Abstract
The diel vertical migration, growth and spawning season of the euphausiid, Thysanoessa longipes, were investigated using seasonal samples collected from waters around the Yamato Rise, central Japan Sea, during the period 1987 to 1999. Thysanoessa longipes was present throughout a broad bathymetric layer extending down as deep as 1000 m. There was a clear trend for larger specimens to occur at deeper depths. The peak of abundance of the total population occurred at depths of 30–300 m at night, and 150–500 m during the day, and the distance of the diel vertical migrations of the total population was estimated to be between 100 and 150 m. Population structure analysis revealed the occurrence of three cohorts aged 0+ ,1 + and 2+ years, with females attaining a larger body size than males. Growth as determined by body length was found to fit well to the von Bertalanffy growth equation. The estimated life span for males and females is 3 years, and females reach maturity in 2 years. Based on the occurrence of calyptopis larvae, spawning of T. longipes was estimated to occur over only a limited period of the year between April and May.
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