Age Composition of Japanese Spanish Mackerel Scomberomorus niphonius (Cuvier 1832) Caught off Hyogo Prefecture, South-western Sea of Japan, as Determined by the Otolith Cross-section Method

The age composition of Japanese Spanish mackerel Scomberomorus niphonius (Cuvier 1832) caught off Hyogo Prefecture, south-western Sea of Japan, was investigated based on the otolith cross-section method. The age analysis demonstrated that the method was useful to clarify and accurately determine age of Japanese Spanish mackerel. The method revealed that annual structures on otoliths were mainly formed in June, which was closely associated with the spawning peaks in the East China Sea. In the Sea of Japan, a large proportion of the catch consisted of age 0 fish, of which most of the fork length (FL) modes were from 350 to 500 mm. The ratio of males and females was in equal proportion in the age 0 fish, but the female ratio increased in the age 1 group and older fishes. The increase of the female proportion in older fish could be related to differences in the age of maturity and migration for spawning. In the southwestern Sea of Japan, age 0 fish that were first caught in September or October were all larger than 250 mm FL, suggesting that fish caught in this area hatch around June and migrate from another area in autumn after growing to more than 250 mm FL.