The main objective of WKNephrops was to review reference points for a range of Nephrops stocks taking account of updated methods and new data (including discard survival rates). While good progress was made in terms of documenting and testing current methodologies for estimating reference points, for the most part further work is need before new reference points can be pro-posed and agreed.
The current approach for basing advice for fishing opportunities for Category 1 Nephrops stocks where on UWTV surveys and FMSY harvest rates derived from per-recruit proxies based on either F0.1, F35% or Fmax (dependent on the perceived stock productivity and apparent vulnerability to overfishing) has been in place since 2009. An evaluation and review of this approach is timely, particularly since issues have arisen for some stocks where the SCA population estimates differs substantially from the UWTV surveys. These differences in population estimates can be as much as an order of magnitude and this appears to be an issue of scaling rather than estimation error. Currently, there does not appear to be a sound explanation of why the two approaches differ so markedly. Work carried out at the meeting on some Nephrops FUs using the SCA approach in a semi-dynamic way suggested that the population signal in the survey was reflected in the catch data, this offers some reassurance that an integrated modelling approach with the surveys as a relative index may be appropriate.
Re-estimation of reference points was carried out for a number of Nephrops stocks and this showed that for most stocks there was little change in reference points over time, whereas for others there have been larger variations. The group did not recommend revising any reference points at this stage until further work has been carried out. For the Bay of Biscay stock (FU23–24)the group recommended that the discard survival rate should be revised and a 50% valueshould be used in future assessments and advice.
Another current issue is that new studies on discard survival raise concern about the discard survival assumptions used in the ICES assessments and advice. This is an issue of importance for advice users since as it affects the way the fisheries are managed, and there is a perception that differences in assumed and actual survival rates could lead to inaccurate ICES advice. A sensitivity analysis was carried out for several stocks at WKNephrops. It appeared that FMSY proxies based F0.1 and F35% were relatively insensitive to the discard survival rate and that FMAX was more sensitive to discard survival rates. Discard survival will affect the shape of the yield per recruit curve, and may influence the estimates of FMSY and F0.1 differently. Further work is required to understand the exact mechanisms. For fisheries where discards account for a very low percentage of the catch and F0.1 or F35% are used as the FMSY proxy, discard survival is not likely to be important. Conversely, for stocks where FMAX is used as the FMSY proxy and which have a high discard percentage FMSY may need to be re-estimated using the best available esti-mate of discard survival.
For stocks that have limited length frequency data and little or no UWTV surveys, the working group is considering a range of DLS methods that rely more heavily on life-history theory in the absence of data on stock abundance. These include length-based indicators, methods based on spawning potential ratio (SPR) and surplus production models such as SPiCT (Pedersen and Berg, 2016). These are established methods within ICES and it is appropriate to investigate their utility for Nephrops. As much of the analysis was being performed at the meeting it was not possible to draw any general conclusions. In one stock investigated, a variety of such data-lim-ited methods gave different estimates of stock status which at face value indicates that reference points are model-sensitive which means that choosing one approach may not be sufficiently ro-bust for management purposes. It was important and useful to see such differences, as model uncertainty is typically under-estimated in ICES assessments.
There is still much work to do in relation to the assessment and derivation of reference points on Nephrops stocks. The move toward dynamic length-based models integrating the UWTV surveys is desirable and may help address the reference point issue. A number of integrated models of this type are already available and include, Stock Synthesis, CASAL, LIME and Gadget. While some work is ongoing on some of these models, a co-ordinated work plan setting out a system-atic approach to the development, testing and application of the methods might be beneficial in the medium term. Where data-limited methods are being examined there may be value in testing these on data-rich stocks to gain an understanding of their performance where stock status is better understood so that the choice of method to apply is well informed.