1Wakkanai Fisheries Research Institute, Wakkanai, Hokkaido 097-0001, 2Soya Fishery Cooperative, Wakkanai, Hokkaido 098-6755, 3Central Fisheries Research Institute, Yoichi, Hokkaido 046-8555, Japan
Seasonal changes in sexual maturity of the Pacific giant octopus Enteroctopus dofleini from the Soya/La Perouse Strait were examined in order to determine a biological index for stock assessment. In addition, the body weight and organ weight of more than 100 specimens were measured at monthly intervals from October 2003 to December 2004. Comparisons of body and gonad weight in females revealed the existence of two groups in the period between autumn and spring. The group with the relatively heavier gonads consisted of mature females containing spermatozoa. A similar relationship was observed in male octopuses between autumn and winter. The group with the heavier gonads consisted of mature males with spermatophores. Based on the assumption that the spawning season was in early summer, the stage of maturity could be used as a biological index for assessing the spawning biomass of Pacific giant octopus in this area.