Size and Age Composition of Southern Bluefin Tuna (Thunnus maccoyii) in the Central Indian Ocean Inferred from Fisheries and Otolith Data
Resource
Zoological Studies, 47(2), 158-171
Journal
Zoological Studies
Journal Volume
47
Journal Issue
2
Pages
158-171
Date Issued
2008
Date
2008-03
Author(s)
Abstract
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.
Subjects
Southern bluefin tuna
Thunnus maccoyii
Size composition
Otoliths
Length-weight equation
SDGs
Other Subjects
Scombridae; Thunnus maccoyii
Type
journal article
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