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Effect of water temperature on gonadal development and emaciation of Japanese common squid Todarodes pacificus (Ommastrephidae)

In this study, the site of spawning ground and the relationship between oceanographic conditions and gonadal development and emaciation of the Japanese common squid Todarodes pacificus were examined based on data from research cruises. The research cruises were conducted in the south-western part of the Sea of Japan and the East China Sea in the winter of 1999. Sea surface temperature (SST) ranged 12.7–23.2°C in the survey area. Mature females concentrated on the continental shelf of the East China Sea, suggesting a major spawning ground was formed in this area. Gonadal development of the collected squid was examined using ovary somatic index (OSI) and oviduct somatic index (ODSI) in females, and emaciation was examined using mantle muscle somatic index (MI) and hepatosomatic index (HSI). Mean OSI and ODSI increased with SST at the stations where SST was greater than 15°C, particulary where SST ranged between 15 and 18°C. In contrast, mean MI and HSI decreased with increase in SST at the stations where SST was above 15°C. These results indicate that female gonads develop rapidly at SST above 15°C, and at the same time adult squids tends to be emaciated at this temperature condition.