Age and growth of longtail tuna, Thunnus tonggol, in the waters off eastern Taiwan was examined based on daily age estimation with otolith increment of fish. Length (fork length, FL) and weight (W) data and otolith samples were collected at the fishing ports of Hualien and Shinkang (eastern Taiwan) which were caught by set net and inshore longline between December 2005 and November 20009. In total, 154 longtail tuna were collected, species ranged from 31.5 to 74.5 cm FL (0.45 to 7.1 kg).
The linear relationships between fish length (FL) and otolith length (Lo), FL=0.0797Lo－7.8462 (R2=0.761), and fish weigh (W) and otolith weight (Wo), W= 0.2603Wo－0.6605 (R2= 0.879), this result revealed that the growth of longtail tuna has a strong relationships with otolith length and weight. The applicability of these relationships in age determination of longtail tuna was investigated. Microincrements, assumed as daily deposite, were observed on transverse sections of sagittal otoliths. The primarily results showed the longtail tuna in the waters off eastern Taiwan were all young of the year (YOY) species. The increments will be count to determine age and establish the growth curve.