Temporal Dynamics of Reproductive Activity and Sexual Group Maturity of Octopus minor in the Deukryang Bay, Southern Coast of Korea

This study was performed to obtain basic information for the resource management and conservation of Octopus minor, one of the important fishery resources in Korea. O. minor used in this study were collected monthly from May 2022 to August 2023 using a drum-shaped net in Deukryang Bay, southern coast of Korea. The overall sex ratio (female:male) of O. minor was 1:1.03 (n = 322; females = 159, males = 163), with a significantly higher proportion of males at a total weight of 150.1 g and above (p < 0.05). The gonadosomatic index (GSI) of females reached its highest value in July, whereas the reproductive complex index (RCI) showed its lowest value. In males, the RCI reached its highest values in June and July, showing a variation pattern opposite to that of the GSI. Although the timing of changes in GSI, RCI, and gonadal morphology and histology differed between females and males, both sexes exhibited clear seasonal patterns. The main mating and spawning season was estimated as summer (July–August). The oocyte development pattern was synchronous, while the spawning pattern was monocyclic with strict seasonality. The size at 50% sexual group maturity of O. minor was estimated at a total weight and mantle length of 49.5 g and 43.6 mm in females, and 49.7 g and 41.9 mm in males. The information on the main spawning season and size at 50% sexual group maturity obtained from the O. minor population in Deukryang Bay, Korea, can be used to establish a prohibited fishing season and size limit for resource management.