This project was developed in response to calls from the SBT Trilateral (now the CCSBT)  Scientific Committee for the development and validation of techniques for the direct estimation of age and growth in the species. Since the early 1980’s, the stock assessment methods used by Australian, Japanese and New Zealand scientists to examine the status of SBT stocks have depended heavily on accurate data on the age structure of the catch, and population. However, there was significant concern that the methods being used to estimate the age composition of the catch - which were based principally on the conversion of lengths or weights to age using growth curves derived from tagging data - were not accurate, particularly for large fish. As the conversions produced indirect estimates of age they could not be validated. The resulting  uncertainty surrounding their accuracy introduced significant uncertainty into the assessments. Given the serious concerns over the status of the SBT stock, which is considered to be at historically low levels, a new approach that allowed direct and validated estimates of age was urgently required.

In 1992, with funding from the FRDC, CSIRO and the Japanese Marine Research Agency, JAMARC, we began a large and integrated project designed at developing and validating new techniques for directly estimating the age and growth of SBT. A large mark and recapture experiment was initiated within a JAMARC-CSIRO tagging program and over the four years 20,204 were injected with strontium chloride (SrCl2); otoliths, scales and vertebrae collected by CSIRO, Australian Fishing Zone and RTMP Observers, and scientists from the Japanese National Research Institute for Far Seas Fisheries were prepared using adaptations of techniques reported in the literature; a relational data base was developed allowing all of the age and growth data produced by the project to be integrated into the central CSIRO SBT data base, the key repository for data used in the annual CCSBT assessments; a collaborative project was developed with Dr John Kalish and his team at the Australian National University using a new technique based on bomb radiocarbon chronometry to provide independent validation of our age estimates for large fish; and samples from our study were sent overseas for review by international experts. 

The project met all of its objectives and has undoubtedly been a major success.  Our findings revealed serious errors in the historical data used within the Tri-lateral/ CCSBT assessments (see Table 1) and the new data produced by the project  have had an immediate and significant impact on both the assessment process and our understanding of the SBT population. The new techniques and data have been accepted by the CCSBT Scientific Committee, prompting an agreement at the 1996 CCSBT meeting to introduce a fishery-wide program of sampling and routine age estimation over the next few years.  

In addition to the achievements made within the project, the new techniques we have developed and validated have the potential over the next few years to resolve many of the outstanding uncertainties surrounding the population biology and demographics of the SBT populations.

Keywords: Age, Southern Bluefin Tuna