The critical requirement for accurate assessment of SBT stocks is recognised internationally. In 1995-96, the SBT fishery was worth $100 million to Australia and $1 billion globally. However, in the same year the species received a CITES 2nd Appendix listing (a response to judgements that it is overexploited and endangered) and Japan proposed a large-scale experimental fishing program (EFP) based on an increase in quota/catch.
The diametrically opposed positions represented by CITES and the Japanese EFP result from fundamental problems in the stock assessment process. Uncertainties within the assessments - in the data on which they are based, in key biological parameters and in the interpretation of historical changes in catch, effort and population parameters - provide the scope for radically different interpretations of results from assessment models. It is likely that as long as these uncertainties remain, there will be the scope for interpreting the data “as it suits”.
At the 1996 CCSBT Scientific Committee meeting, the significant uncertainties within the assessment process were identified. The proposed project addresses four of the high priority areas:
1. Accurate and validated age-length keys based on direct age estimation data are required for improving the catch-at-age matrices that are used as the basis for VPA’s.
2. Accurate estimates of the age at first maturity are required for establishing the extent of the parental biomass.
3. Accurate estimates of the nature and extent of changes between the 1950’s and 1990’s in the growth of 1-4 year old SBT are required for understanding the effect of these on current and past assessments.
4. Estimates of the natural mortality of 8-40 year old SBT are required for tuning of the VPA’s and stock projections.
The project addresses a number of items in the Southern Tuna MAC 5-Year Research Plan and has been endorsed by the MAC as a high priority.
Final report
The CCSBT has recognized for a number of years that a better understanding of the population biology and demographics of southern bluefin tuna (SBT) is necessary for improved population modeling and stock assessments. In 1996, the CCSBT Scientific Committee identified three areas where our understanding of SBT biology was inadequate: catch-at-age, age-at-maturity and natural mortality. The Scientific Committee also highlighted the need to measure changes in the growth rates of juvenile SBT over the past 30-40 years. This project was developed in response to these concerns.
Determining the age structure of the SBT catch is a basic requirement for understanding the species population dynamics and undertaking age-based stock assessments. Validated ageing techniques have only recently been developed for SBT and a previous FRDC project (Gunn et al. 1996 - FRDC 92/42) estimated age for approximately 1,000 SBT, collected predominantly off Tasmania and in the Great Australian Bight (FRDC 92/42). During the current study, we selected a further 2000 SBT otoliths for ageing, and use these data in conjunction with those from FRDC 92/42 to complete our objectives.
To estimate and compare the age composition of catches in each of the major SBT fisheries (Objective 1), we developed several age-length-keys for SBT (Objective 3), and applied them to catch-at-length data to estimate the age composition of catches for each fishing ground/fleet. Two age-length-keys were developed based on samples collected from the Indonesian fishery on the spawning ground (separate key for each of two spawning seasons) and two for SBT caught south of the spawning ground (separate key for each sex).
Our results show that the distribution of ages within the catches varies significantly among areas and fleets. The Australian summer surface fishery in the GAB was dominated by 2 to 4 year-old SBT, while the Taiwanese winter longline fishery across the central Indian Ocean was dominated by 3 and 4 year-olds. This is consistent with archival tagging work that has shown that juveniles tagged in the GAB in summer undertake annual feeding migrations into the Indian Ocean during winter before returning to the GAB in early spring (Gunn and Block, 2001). Data collected from Taiwanese longliners transhipping in Mauritius show that 3 and 4 year-old SBT are also caught off south-east Africa from November to February, indicating that not all juveniles spend summer in the GAB. Although a few SBT as old as 7 years are caught in the Taiwanese fishery (30-35°S), it appears that adult SBT do not generally forage this far north.
Keywords: Southern bluefin tuna, age distribution, age-length keys, sexual dimorphism, age-at-first maturity, growth rates, natural mortality.
