| Research Abstract |
Reproductive strategy related to stock fluctuation on the ommastrephid squid Todarodes pacificus are examined by field survey and captive experiments. Immature squid were collected and maintained in a raceway tank where they matured, mated, and spawned. Gelatinous egg masses were spherical and nearly buoyant. The largest egg mass measured 80 cm in diameter and contained approximately 200,000 eggs. The egg-mass surface layer effectively prevented crustaceans, protozoans, and bacteria from infesting the masses. Paralarvae hatched after 4-6 days at 18-19℃ and actively swam at once, with many individuals swimming at the surface. Egg masses disintegrated soon after hatching. Survival of eggs and paralarvae were examined at temperatures ranged from 3.5-29℃ to determine the optimum temperature range for development and survival. Highest embryonic survival rates occurred between 15 and 23℃. It is sugggested that T. pacificus spawns in waters warmer than 15℃, and egg masses maintain their struc
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ture for about 4-9.5 days before disintegrating at temperatures between 15 and 23℃.
The inferred spawning sites of T. pacificus around Japan are assumed that egg masses and hatchlings occur at temperatures between 15 and 23℃, and above the continental shelf and slope, because captive females regularly sit on the tank bottom just before spawning. Annual catches of Todarodes pacificus in Japan have gradually increased since the late 1980s. Paralarval abundances have also been higher since the late 1980s than during the late 1970s and mid-1980s. We propose a possible scenario for the recent stock increase based on changing environmental conditions. First, we examined trends in the annual variations of stock and larval catches, and infer potential spawninig areas, assuming that egg masses and hatchlings occur at temperatures between 15 and 23℃, and over the continental shelf. We then infer changes in the spawning areas during 1984-95 based on GIS data. We conclude that since the late 1980s the fall and winter spawning areas have overlapped in the Tsushima Strait and near the Goto Islands, and that winter spawning sites have expanded over the continental shelf and slope in the East China Sea. Less
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