Archiving of hardparts for routine ageing and developing age-length keys for the Australian SBT surface fishery 2015-16

[摘要] This report provides an update on the archiving of southern bluefin tuna (SBT) otoliths sampled in Australia over the last year (2015/16 fishing season), and the estimation of age and proportion-at-age of the Australian SBT surface (purse-seine) fishery for the 2014/15 fishing season.Otoliths from 171 SBT were received and archived into the CSIRO hardparts collection during the project. Of these, 137 were sampled from the Australian surface fishery (Port Lincoln, South Australia) and 34 from CCSBT gene-tagging fieldwork operations in the Great Australian Bight. As noted previously, the current otolith sampling protocol for the surface fishery 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 led 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 2014/15, 100 otoliths were selected and sent to Fish Ageing Services Pty Ltd. Age estimates ranged from 1 to 6 years. Proportions at age were estimated using standard age-length-keys and by applying the method developed by Morton and Bravington (2003) (MB 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 MB estimator with “unknown growth” (see Methods) should be most accurate. For the 2014/15 season, the proportion at age estimates from the MB method with unknown growth are 73% age 2 and 20% age 3. These estimates are very similar to the 2013/14 season, but suggest a larger proportion of age 2 and smaller proportion of age 3 fish in the catches than in any of the previous seasons.The results of the project will be presented as working papers at the annual CCSBT Extended Scientific Committee (ESC) meetings in September 2016.

[发布机构] CSIRO

[学科分类] 地球科学(综合)