Inter-benchmark to revise the advice framework for the Sprat stock in 7.de based on the most recent changes to data-limited short-lived species assessments
reportposted on 2021-03-18, 00:00 authored by ICESICES
The workshop aimed to review conclusions of the Workshop for Data-Limited Stocks of Short-Lived species (WKDLSSLS) and determine the most appropriate advice framework for Sprat in divisions 7.d and 7.e. Previously, advice has been provided by applying a ratio of previous sur-vey index year values to the preceding years catch advice, with application of a 20% uncertainty cap (UC) and precautionary buffer (PB).Previous work recommended several assessment approaches, including “1 over 2” ratio-based advice with 80% UC; surplus production model (SPiCT); and Constant Harvest Rate (CHR), de-rived from Management Strategy Evaluation (MSE). These approaches were further evaluated, with SPiCT results being too uncertain currently to consider further.The MSE was tuned to the stock’s biological variables, from survey observations or borrowing from North Sea sprat.Three MSEs scenarios, relating to survey “catchability” were run, assuming no, 50% and 100% overestimation in survey biomass. Under each scenario, the maximum precautionary CHR per-formed better in terms of associated risk and harvest than “1 over 2” ratio-based advice. Result-ing CHRs were 19%, 12% and 10% (No, 50% and 100% overestimation respectively). Assuming that some overestimation may take place in the survey, the CHR assuming 50% overestimation was adopted.Processing of growth within the MSE results in within-year age class growth being unaccounted for. This results in underestimation of stock weights at the time of the survey, and so over esti-mation of realised CHR. A correction factor to account for this was calculated at 0.714. Applied to the CHR (12%) this results in a corrected CHR of 8.57% to be applied against most recent survey biomass index to define catch advice for a management year running July to June. If applied from January to December, a review of the advice after completion of the October/ No-vember survey is recommended in the interim year, as a safeguard to advice still being applica-ble. The CHR is more risk averse than the previously applied 1 over 2 ratio-based advice.As the survey biomass index is available in November, it may be possible to move provision of advice to November, for the execution of the fishery from January to December.The IBP concluded that:1. CHR based advice is more appropriate than 1 over 2 ratio-based advice.2. Advice should be applied to the fishery from July year y to June year y+1.3. A CHR of 8.57% is risk averse and provides highest yield.4. If application of advice from January to December is to be maintained, it should be re-viewed when survey data become available following the October/ November survey.5. The optimal approach would be to undertake the assessment at the end of the October/ November survey (year y) in time for application from January to December (year y+1).
Published under the auspices of the following ICES Steering Group or Committee