Workshop on Data-limited Stocks of Short-lived Species (WKDLSSLS2)

MSE testing of harvest control rules based on trends in biomass indices were analysed for generic anchovy-, sprat-, and sardine-like stocks, including several operating models. Consideration wasgiven to coupling with different Uncertainty Cap (UC) levels and biomass safeguards, and the application of simple constant harvest rates.

With regards to the coupling in time between assessment, advice, and management: results from WKDLSSLS1 were confirmed, indicating that the shorter the lag between observations, advice,and management, the bigger the catches and the smaller the risks, whereby in-year advice shouldalways be preferred over the usual calendar (with an interim year) advice.

Rules of type 1-over-2 outperform 2-over-3 (also called 1o2 and 2o3 respectively). When appliedalone (without any Uncertainty cap or Biomass safeguard) for the sprat 7.de-like stock, it wasshown that the 1o2 rule is capable or reducing risk faster than the 2o3 and reaching levels below0.05 in the long term, while the latter does not. In addition, the simulations on anchovy andsardine-like stocks show that even after combining these rules with symmetric and asymmetric Uncertainty cap levels, rule 1o2 results in smaller risks for the same catch levels as the 2o3 rules,given a common Uncertainty cap level, which indicates that 1o2 outperforms 2o3 rules for theseshort-lived species.

For all operating models, it was found that the 1o2 rule with symmetric UC(-0.8,0.8) impliesfaster reduction of risks than for any other tested UCs (particularly in the medium term), thoughat the expense of greater reductions of yield. For almost all Operating Models (OMs), the 1o2rule with 20% cap was the least precautionary option. In general, inclusion of a biomass safeguardremarkably reduces risk in the medium and long terms by slightly reducing the relative yields for the stocks that have been historically over-exploited. A biomass safeguard based on I_stat (geometricMean(I_hist ) ^. e^{-1.645.sd(log(1hist))}) is proposed due to the greater robustness to the length of historical observations.

Application of both an uncertainty cap and a biomass safeguard (Istat) to the 1o2 rule appears toperform better across all OMs and time-scales than either mechanism on its own. For short-livedstocks presumed to have been subject to an exploitation level before management at or above proxy FMSY levels the 1o2 rule with 80% symmetric uncertainty cap and with biomass safeguard (Istat) is the preferred option due to the faster reduction of risk levels in the first ten years (mediumterm). However, it should be noted that for stocks, which have likely been lightly exploited inthe past, other rules may show a better balance between catches and risks. Hence, an earlierassessment of the past exploitation of the stock is very relevant to select the most suitable HCR for the management.
Application of constant harvest rate rules can maintain constant risks, but are not able to move the stock towards precautionary levels when starting from high risk status, therefore, they require careful analysis of sustainable reference levels of harvest rates. Global comparisons suggest that when a careful tunning of a sustainable constant harvest rate is made by taking into account the stock life history and catchability and CV of the observation index, then such a constant har-vest rate strategy will result in higher sustainable catches for the standard allowable levels of risks (0.05). However, if such a tuning is not achievable because of poor knowledge of the stock or of the observation properties, then the WK recommends for short-lived small pelagic fish stocks, the former trend rule 1o2 with a symmetric Uncertainty Cap constraint of 80% and with Biomass safeguard (Istat). However, due to the catch reduction properties it has, this trend rule should be considered a provisional HCR with the aim of achieving a better management system in about ten years or earlier. Longer application may lead to major losses of catches to the fishery in the long term.
The work of WKDLSSLS is considered unfinished. Further research on the definition of optimal harvest control rules for data-limited short-lived stocks is ongoing. Therefore, the suggested either tuned constant harvest rate or the trend rule should be taken as an interim (provisional) proposal while guidelines are refined in 2021.