Strontium: calcium ratios in statoliths of Japanese common squid Todarodes pacificus (Cephalopoda: Ommastrephidae) as indicators of migratory behavior

摘要

Ontogenetic variation in the strontium:calcium (Sr:Ca) ratios was measured in statoliths of the Japanese common squid Todarodes pacificus. Two different geographical groups of T. pacificus inhabit the Sea of Japan; these groups are separated by the Subarctic Front during the summer. Element ratios were measured with a wavelength-dispersive spectrometer for 11 individuals from a cold-water subarctic region, which harbors large squid (Subarctic Group), for 12 individuals from a warm-water region, which harbors smaller squid (Tsushima Group), and for 2 individuals from the Subarctic Front. The squid ranged from 190 to 218 mm in mantle length (ML) and were 191 to 262 d old. Similar measurements were obtained for 12 tagged T. pacificus (187 to 259 mm ML; 214 to 312 d old) that were released and recovered in the Subarctic and the Tsushima Current between June and August 1997 (tagging duration 5 to >40 d). Ontogenetic variation in Sr:Ca ratios from hatchlings to sub-adults differed between the 2 geographical groups. Sr:Ca ratios were high at hatching followed by a drastic decrease up to 10 d of age for both groups. These ratios increased until 40 d of age, continuing at high levels until around 150 d, when they decreased in all specimens of the Tsushima Group. In the Subarctic Group, however, the variations in Sr:Ca ratios differed between individuals. Sr:Ca ratios at the age when squid recruited to either the Subarctic or the Tsushima Current were similar. These differences reflect the different spawning grounds and transport routes of the 2 groups. Thermal history was recorded from the statoliths of tagged individuals, and small fluctuations and seasonal variations in the Sr:Ca ratios were observed. These correspond to diel vertical movements and endogenous rhythms of the squid, and the formation and movements of the cold-water layer in their habitat.