Temperature‐Driven Changes in Spawning Characteristics of the East Asian Common Octopus <i>Octopus sinensis</i> in the Central Seto Inland Sea, Japan

ABSTRACT The East Asian common octopus Octopus sinensis is distributed in the coastal waters of East Asia and can grow to around 2 kg in 12–18 months. This species is semelparous, and its spawning season is highly variable and depends on the water temperature, which has increased in the Seto Inland Sea, western Japan. To investigate the impact of this temperature increase on the reproductive characteristics of O. sinensis , we collected 3387 and 757 samples using small trawls in 1999–2001 and in 2019–2021, respectively, and compared the body condition of all samples and the occurrence of final maturation stage females between the two sampling periods. Paralarval abundance was also compared using samples collected with a plankton net. The relative condition factor indicated that individuals were in a worse growth condition in 2019–2021. The mean mantle length of final maturation stage females was significantly larger in 2019–2021; however, there was no significant difference in body weight between the sampling periods. Seasonal changes in the gonadosomatic index of females and the occurrence of final maturation stage females between the sampling periods indicate that the proportion of final maturation stage females peaked in September during 1999–2001 but shifted to spring and late autumn in 2019–2021. Paralarval densities in October were more than 10 times higher in 1999–2001 than in 2019–2021, which may be related to the body condition and changes in the spawning season due to increasing water temperatures.