Reproductive traits of the diamond squid Thysanoteuthis major around Okinawan waters, south Japan, during the fishing season

Abstract

Thysanoteuthis major is a large squid that inhabits tropical and temperate regions and serves as an important fishery resource in the Northwest Pacific. To clarify the basic biological characteristics necessary for stock management, we examined the spawning season, sexual maturity, and 50% maturity size of T. major in the Okinawan waters using gonadal histology. Mature females of this species were observed to occur during the fishing period from December to May, with an estimated peak between February and April. The ovaries exhibited asynchronous oocyte development, suggesting multiple spawning events throughout their lifetime. The minimum maturity size was 574 mm mantle length (ML) for females and 492 mm ML for males, whereas the 50% maturity length was 664 mm ML for females and 500 mm ML for males. These findings provide a basis for evaluating effective stock management measures for T. major.