Zoological Studies

Vol. 47 No. 2, 2008

Size and Age Composition of Southern Bluefin Tuna (Thunnus maccoyii) in the Central Indian Ocean Inferred from Fisheries and Otolith Data

Jen-Chieh Shiao1, Shui-Kai Chang2,*, Yu-Ting Lin3, and Wann-Nian Tzeng3

1Institute of Oceanography, College of Science, National Taiwan University, 1 Roosevelt Road, Sec. 4, Taipei 106, Taiwan
2Institute of Marine Affairs, College of Marine Sciences, National Sun Yat-Sen University, 70 Line-Hai Road, Kaohsiung 804, Taiwan
3Institute of Fisheries Science, College of Life Science, National Taiwan University, 1 Roosevelt Road, Sec. 4, Taipei 106, Taiwan

Jen-Chieh Shiao, Shui-Kai Chang, Yu-Ting Lin, and Wann-Nian Tzeng (2008) Estimating the age of the commercial catch of southern bluefin tuna (SBT, Thunnus maccoyii) is essential to the age-based stock assessment process and an understanding of the population dynamics of the species.  This information is, however, very incomplete for the central Indian Ocean (CIO), where only the Taiwanese fleet catches SBT, and will be influential in providing an overall picture of the recently suspected weak recruitment that emerged from several fishery indicators in the SBT's nursing and feeding grounds.  Based on commercial logbook data of Taiwanese longliners with the size of each SBT attached, this report provides age composition information from 2002-2005 with annual mean ages of 4.3-5.0 yr, through length-to-age conversions by cohort slicing procedures.  The commercial length distributions have been verified by those obtained from 10 trips of scientific observations, and the age compositions were compared with those estimated from directly aging 473 otolith samples collected by those observers.  About 90% of the total SBT catch was < 8 yr old, the presumed youngest age at first sexual maturity.  Thus, SBT caught in the CIO may consist mostly of sexually immature individuals and showed no geographical trends.  Four yr of age and size composition data indicated a stable migrating SBT stock in the CIO, compared with the weak recruitment of young fish to the southern Australian nursing grounds and New Zealand feeding grounds during the same period.

Key words: Southern bluefin tuna, Thunnus maccoyii, Size composition, Otoliths, Length-weight equation.

*Correspondence: Tel: 886-7-5252000 ext. 5303.  Fax: 886-7-5250050.  E-mail:skchang@faculty.nsysu.edu.tw