School whiting (Sillago flindersi) stock assessment based on data up to 2016

[摘要] This document updates the 2009 assessment of school whiting (Sillago flindersi) to provide estimates of stock status in the SESSF at the start of 2018 and describes the base case assessment and some of the issues encountered during development. This assessment was performed using the stock assessment package Stock Synthesis (version V3.30.08.03). The 2009 stock assessment has been updated with the inclusion of data up to the end of 2016, comprising an additional eight years of catch, discard, CPUE, length and age data and ageing error updates. A range of sensitivities were explored.A preliminary base case was presented at the September SERAG meeting and a provisional base case at the November SERAG meeting, with improvements to the balancing of the conditional age-at-length in the provisional base case and incorporating fixes to a bug discovered in Stock Synthesis in the interim. Following the November SERAG meeting, the November provisional base case was updated by changing the spawning month from July to January, at the request of SERAG, and a further variation was produced with improvements to the estimated growth curve, again with January spawning. This gave a choice of 3 fully balanced alternative base cases to be considered by the SERAG.Under these three alternative base cases, the current spawning stock biomass ranges from 47% to 50%. When the final base case is chosen, the 2018 recommended biological catch (RBC) will be made available, calculated under the agreed 20:35:48 harvest control rule. Similarly the long term RBC will also be made available.Exploration of model sensitivity showed variation in spawning biomass across all sensitivities ranging from 39% to 57% of SSB0 with greatest sensitivity to age at 50% maturity. A preliminary sensitivity removing all catch data north of Barrenjoey Point resulted in a depletion of 17%, but the resulting estimate of mortality was unrealistically low. This sensitivity was repeated with mortality fixed at 0.6, corresponding to the fixed value for mortality used in the 2008 assessment. Balanced sensitivities with winter rather than summer spawning, and with improved growth estimates produced very similar results to the SERAG proposed base case with summer spawning.Changes to the 2009 stock assessment include: separating length frequencies into onboard and port collected components, with a joint selectivity pattern estimated; weighting length frequencies by shots and trips rather than fish measured; and using the latest agreed best practice tuning method. The updated assessment is remarkably consistent with the results from the 2009 assessment, despite an additional 8 years of data, improvements to data processing and modifications to Stock Synthesis.

[发布日期] 2017-12-07 [发布机构] CSIRO

[学科分类] 地球科学(综合)