Interannual changes in body size and hatching period of Japanese common squid Todarodes pacificus off the Pacific coast of Hokkaido

Abstract

The body sizes of Japanese common squid Todarodes pacificus, which migrated to waters off the Pacific coast of Hokkaido, exhibited long-term changes. It was hypothesized that the long-term body size changes corresponded to population dynamics and regime shift. In addition, the body-size changes might be attributed to changes in the hatching period. In the present study, we examined a long-term trend in body-size using the specimens that were collected off the Pacific coast of Hokkaido from 1965 to 2008. In addition, the relationship of body sizes to hatching dates was examined for the periods from 1999 to 2008. The annual mean body size tended to be larger during 1974–1984 than duringthe earlier (1965–1973) and later (1991–2008) periods. Such a decadal trend of body sizes seemed to correspond tothe long-term trends in catch per unit effort and the regime shift. The hatching dates during 1999–2008 ranged fromNovember to April, with a main season extending from February to March. Moreover, a negative relationship was found between the hatching dates and body sizes, suggesting the influence of the hatching period on changes in body-size.