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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
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