Determination of the spawning history of Pacific saury Cololabis saira from rearing experiments: identification of post-spawning fish from histological observations of ovarian arterioles

Rearing experiments under controlled temperature conditions were carried out to establish a methodology for estimating the spawning history of the Pacific saury Cololabis saira. The experiments involved histological examination of ovarian arterioles at various maturational stages. We found that pre-spawning immature fish lacked Victoria blue (VB)-positive arterioles in the ovaries, as did maturing fish, while post-spawning females had VB-positive arterioles. The latter were also observed after 6 months when most spawning activity had ceased. The results indicate that VB-positive ovarian arterioles are a characteristic feature of actively spawning Pacific saury and that females with VB-positive arterioles can be classified as post-spawning fish. Females without VB-positive arterioles can be regarded as “practically unspawned” fish, and this group may include some fish that have spawned a number of times, in addition to completely unspawned fish. The methodology has many practical uses such as in the estimation of maturation rates for stock management.