Catch fluctuations of the diamond squid Thysanoteuthis rhombus in the Sea of Japan and models to forecast CPUE based on analysis of environmental factors

This paper describes recent catch fluctuations of the diamond squid Thysanoteuthis rhombus in the Sea of Japan and the development of models used to forecast catch per unit effort (CPUE) off Hyogo Prefecture based on statistical analyses of environmental indices measured 600 km upstream in the Tsushima Current. Annual catches during 1989-2002 fluctuated widely, and prefectural annual catches were closely related to each other, especially among the western prefectures. Four indices in June were closely related with CPUE during the fishing season (September-November) off Hyogo: (1) water temperature in the Tsushima Strait, (2) salinity in the Tsushima Strait, (3) sea level at Izuhara (Tsushima Island) and (4) sea level difference between Izuhara and Hakata (Kyushu Island). Using these indices as independent variables, simple and multiple regression analyses were conducted, and CPUE was accurately estimated by both regression and extrapolation, indicating that the CPUE off Hyogo can be forecasted 2 months before the fishery starts using the described models. The strong correlation among catches in different prefectures suggests the indices discussed here affect the catches over a large area.