Daytime surface swarms of the euphausiid Thysanoessa inermis off the west coast of Hokkaido,northern Japan

Biological characteristics of the daytime swarming of the euphausiid Thysanoessa inermis (Krøyer) were examined off the west coast of Hokkaido in the northeast Sea of Japan between 1982 and 1987. Swarms were composed exclusively of fully mature individuals; males possessed spermatophores at the ejaculatory ducts and females had attached spermatophores at the thelycum. Females predominated in most swarms, while males or nearly balanced swarms were infrequently encountered. Fully mature females occurred in the late swarming season, usually mid-April. Examination of stomachs indicated that feeding activity was low during the daytime surface swarming behavior. The body sizes of swarming T. inermis occurring around Yagishiri Island were significantly larger than those found simultaneously off the Shakotan Peninsula. This suggests that the euphausiid populations of the two areas are different.