Workshop on the management strategy evaluation of the reference point, F_cap, for Sprat in Division 3.a and Subarea 4. (WKspratMSE)

The Workshop on the management strategy evaluation of the reference point, F_cap, for Sprat in Division 3.a and Subarea 4 (WKSpratMSE) was held in ICES headquarters in Copenhagen 11-12 December 2018, and continued to work by correspondence through February 2019. The aim of the workshop was to estimate an appropriate Fcap for sprat in Division 3.a and Subarea 4 (Skagerrak, Kattegat, and North Sea) and sprat in divisions 7.d and 7.e (English Channel). This was done in response to the outcomes of the Benchmark Workshop on Sprat (WKSPRAT 2018). Two external reviewers participated and reviewed the analytical work and conclusions of the workshop. See full participant list in Annex 1.
Sprat in the English Channel (Divisions 7de) is a category 3 stock and is assessed based on the acoustic survey carried out in area 7.e. Previously, the “2-over-3” rule had been used to provide catch advice, but evaluations during the last WKSPRAT benchmark suggested that such rule is not appropriate for short, highly productive species. The workshop therefore compared through simulations the performances of a “1-over-2” rule (with and without an uncertainty cap) and of different fixed harvest rates. The simulations suggested that a 1-over-2 rule for short-lived highly productive species might cause the stock to fall below safe levels and eventually to collapse because the rule is not reactive enough to limit the catches when there is a recruitment failure. When removing the uncertainty cap, performances improve, but the risk still remains above safe limits. The use of a fixed harvest rate seems to prevent risk and maximize catch, depending on the proportion used. Simulations suggest that a 20% harvest rate is considered appropriate to maintain the stock at safe biomass levels and to produce relatively high yield. One external reviewer suggested that these rules continue to be explored at the relevant upcoming Workshop for Data-limited Short-lived Stocks (WKDLSSLS).
Sprat in Skagerrak, Kattegat, and the North Sea (Division 3a, Subarea 4) is a category 1 stock and follows the ICES MSY approach where the advised yearly catches correspond to the estimated stock biomass in excess of the MSY B_escapement, but constrained so that the fishing mortality is no higher than a limit F (F_cap). A “full” closed loop management strategy evaluation (MSE) was carried out to test for a range of F’s and find an appropriate F_cap value that showed a less than 5% probability of SSB < B_escapement. The value of F_cap was conditioned on using Bpa as MSY B_escapement. A “short-cut” MSE featuring an assessment emulator was implemented to compare the two approaches. The stock and fleet dynamics were conditioned on the most recent assessment from WKSPRAT2018. Performance statistics between the “full” and “short-cut” MSEs were similar, suggesting that short-cut MSEs should continue to be explored, as they have the advantage of being unaffected by assessment numeric convergence. According to these analyses, F_cap of 0.69 is precautionary.