The potential use of the second dorsal fin rays of skipjack tuna Katsuwonus pelamis as an aging character.

Abstract

Cross-sections of the second dorsal fin rays were used to estimate the age of skipjack tuna Katsuwonus pelamis from the East coast of Japan. For validation of the estimated age, the 19th to 23rd vertebrae centrum were also used to estimate the age. Skipjack tuna were sampled in May, July and September 1985 and from May to November 1986 at Choshi fisheries market (N=230), and from July to October 1986, off Ibaraki (N= 30). The size of the fish ranged from 230 to 624mm in fork length.Zero to Five annulus groups were recognized, and 89% of the number of annulus count on cross-sections of the fin rays, agreed with the number of annulus count found on the vertebrae from the same fish. The mean fork length for the fish from the same annulus group are presented and compared with other works. The advantage of using second dorsal fin ray cross sections in the age analysis of skipjack tuna were discussed and are as follows: 1) they are easy to sample, 2) they do not alter the commercial value of the fish, and 3) they can be used as alternative or to validate other aging methods.