This report provides an update on the archiving of SBT otolith sampled in Australia and Indonesia over the last year (2013/14 fishing season) and the estimation of age and proportion-at-age of the Australian SBT surface (purse-seine) and Indonesian longline fisheries for the 2012/13 fishing season.
Otoliths from 1783 SBT were received and archived into the CSIRO hardparts collection during the project. Of these, 146 were sampled from the Australian surface fishery (Port Lincoln, South Australia) and 1637 from the Indonesian longline fishery (Benoa, Bali). As noted previously, the current otolith sampling protocol in Port Lincoln does not provide either a fixed number of otoliths from each length class or representative samples of otoliths from all length classes in proportion to their abundance in the catch. This has lead to less robust age-length-keys and highlights the need to develop sampling protocols that provide a more representative sample of the size at age for the full size range of the catch for the purpose of constructing annual age-length keys.
To estimate the age of SBT caught in the Australian surface fishery in 2012/13, all otolith sampled (n=96) were sent to Fish Ageing Services Pty Ltd. Age estimates were obtained for 95 SBT ranging from 2 to 8 years. Proportions at age were estimated using standard age-length-keys and by applying the method developed by Morton and Bravington (2003) (M&B method) to the combined age-length data and length frequency data obtained from sampling the catch. Provided that the length frequency data are representative of fish caught in the surface fishery, and given our goal of estimating proportions at age in the catches (not in the population), the M&B estimator with “unknown growth” (see Appendix) should be more accurate. The proportion at age estimates from the M&B method with unknown growth suggest that in recent seasons (2010/11 and 2011/12) there has been a higher proportion of age 2 fish and smaller proportion of age 3 fish than in previous seasons. However, this was no longer the case in 2012/13, with the estimated proportion of fish at ages 2 and 3 (0.16 and 0.75 respectively) returning to levels more commonly seen in the past.
To estimate the age of SBT caught in the Indonesian longline fishery, 500 otolith samples in 2012/13 were sent to Fish Ageing Services Pty Ltd. Age estimates were obtained for 474 SBT ranging from 6 to 36 years. Proportions at age were estimated using a standard age-length-key applied to the length frequency data. The length frequency showed a new mode of small fish at about 145-155 cm FL in both the 2012/13 and 2013/14 seasons. In 2012/13, this mode corresponded to fish aged 7-9 years. Investigations are being undertaken to determine whether the small SBT sampled in 2012/13 and 2013/14 were caught on or off the SBT spawning ground.
The results of the project will be presented as working papers at the annual CCSBT Extended Scientific Committee (ESC) meetings in September 2014.