In relation to assessment methods for short-lived data-limited stocks and estimation of biological and MSY proxy reference points, the WK focused on the application of SPICT (Pedersen and Berg, 2017). The WK was updated on recent improvements of SPiCT and the harvest control rules (HCRs) used to manage stocks after WKLIFE VII and VIII by including either the MSYfractile or MSY-PA rules. For the optimal SPiCT advice rule, users should refer to the update ICES guidelines. Work on fitting SPICT to case studies was made before and during the workshop: Assessments to Anchovy in 9.a South resulted in a satisfactory fitting of SPICT, whilst fits
to Anchovy in 9.a West and to Sprat 7.de were still unsatisfactory. In addition, there were some presentations on applications of SPiCT to several Cephalopod populations. Length-based indicators of stock status were discarded as generally they are not suitable for short-lived species where recruitment induces major interannual changes in the length distribution of catches. A
provisional application of a two-stage assessment was presented for Sprat in 7de, but results were still provisional.
In relation to the evaluation of management procedures for these stocks, MSE testing of harvest control rules based on trends of biomass indices were analysed for anchovy-, sprat-, and sardinelike stocks including several operating models. All simulations showed that the shorter the lag between observations, advice and management, the bigger the catches and the smaller the risk.
This implies that In-year advice should always be preferred over the normal calendar (with an interim) year advice for these stocks. Major drivers of risks are by order of relevance: historical exploitation level (and trajectory), and the harvest control rule (HCR) with its selected Uncertainty Cap (UCap). This emphasizes the relevance of trying an initial assessment of the relative status of the stock regarding optimal exploitation to judge if a precautionary buffer is required to start management. Further work on the assessment of past exploitation level is required.
Regarding the trend-based harvest control rules (HCRs): In general, 1-over-2 outperforms 2- over-3 rule (ICES default rule) because for quite similar catches the former implies lower risks. For symmetrical application of the interannual uncertainty cap, best performance (least risks for minimum reduction of catches) occurs using the 1-over-2 rule with a symmetrical 80% uncertainty
cap. The riskiest performance results were from applying a 20% uncertainty cap, both for 1-over-2 and 2-over-3, and the performance worsens with time. For asymmetrical Uncertainty Caps, tested for rules with a maximum interannual upward revision of 20%, results showed optimal performance when allowing reductions of 60% or greater percentages from the previous
advices for in-year advice, and of 70% or greater for normal (calendar) advice. While the 1-over-2 rule with asymmetric uncertainty cap is the most precautionary, it implies a continued large reduction of catches. The 1-over-2 rule with no uncertainty cap gives the highest catches at all times. Intermediate rules in terms of balance between catches and risks are: 1-over-2 (with symmetrical
80%Ucap) and 1-over-2 with biomass safeguard (using either Imin, the minimum past observed abundance index, or Itrigger, 1.4*Imin). Rule 1-over-2 with symmetrical 80% Uncertainty cap might be preferred as a good compromise between moderate risks and catches though it can lead to major reduction of catches in the long term.
Given the trade-off between competing rules, it seems that selection of a rule should better be made in consultancy with managers and stake holders according to their objectives for each fishery.
Further research will be needed on the definition of proxies for BRPs and of the optimal harvest control rules (including the SPiCT advice rules) for the management of these SLDLS, covering further testing of biomass safeguards and of asymmetric uncertainty caps or the use of constant or variant harvest rate strategies instead of the trend-based rules.
|Publisher||International Council for the Exploration of the Sea (ICES)|
|Number of pages||169|
|Publication status||Published - 2019|
|Series||ICES Scientific Report|