| Co-Investigator(Kenkyū-buntansha) |
BOWER John R. Hokkaido Univ., Field Sci. Cent. For North. Biosph., Assis. Prof, 北方生物圏フィールド科学センター, 助手 (10312406)
ISODA Yutaka Hokkaido Univ., Grad. School of Fich. Sci., Asso. Prof., 水産科学研究科, 助教授 (10193393)
SAITOH Sei-ichi Hokkaido Univ., Grad. School of Fich. Sci., Prof., 水産科学研究科, 教授 (70250503)
YAMAMOTO Jun Hokkaido Univ., Field Sci. Cent. For North. Biosph., Assis. Prof, 北方生物圏フィールド科学センター, 助手 (10292004)
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| Research Abstract |
In short-lived squids, recruitment success most likely depends on the physical and biological environments at the spawning and nursery grounds. No egg masses have been observed in the natural habitat. During 2000-2002, MOCNESS and an ROV were used to examine the distribution of egg masses and paralarvae of Todarodes pacificus in the Sea of Japan. Gelatinous egg masses were observed at two stations in the Tsushima Current along the continental shelf off the Oki Islands in November, 2000. They occurred within the pycnocline (75 and 95 m depth, respectively), where temperatures were about 18℃, which was suitable for embryonic development. Upon hatching, paralarvae may rise to the surface layer, where they would be carried into the convergent frontal zone in the Tsushima Current.
Annual catches of T. pacificus, have markedly increased since the late 1980s, and recent catches have equaled those of the 1960s. The winter spawning area of T. pacificus in the East China Sea shrank when adult stocks decreased during a cool regime that occurred before 1988, and that its fall and winter spawning areas extended and overlapped in the Sea of Japan and East China Sea when adult stocks increased during a warm regime after 1989. Successful hatching from egg masses was to be higher when the mixed layer depth (MLD)is shallower than the bottom depth. We examined the relationship between annual catch of the winter-spawning stock, and wind speed and air temperature in the East China Sea in February during 1980-2000 to determine how a series of calm and warm winters might promote a stock increase related to changes in the MLD. Annual catches markedly increased during a decade of weak wind speed and warm air temperature from the mid-1980s to mid-1990s, suggesting that the strength of winter wind stress affects the winter-spawning stock fluctuations in T. pacificus.
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