Report of the Workshop for management strategy evaluation for Norway Pout (WKNPOUT): 26-28 February 2018, Copenhagen, Denmark

The ICES Workshop for management strategy evaluation for Norway Pout (WKNPOUT) took place 26–28 February 2018 at ICES Headquarter chaired by Andrés Uriarte, Spain, with the assistance of ICES Secretariat. 12 participants, both scientific experts and stakeholders, from Denmark and Norway, attended the meeting.

The group addressed the special request from the European Union and Norway to advise on the long-term management strategies of Norway Pout in ICES Subarea 4 (North Sea) and ICES Division 3.a (Skagerrak-Kattegat). The proposed management strategy is based on the ICES escapement strategy with the aim of achieving a high probability of having the minimum SSB required to produce MSY (Blim) surviving to the following year.

ICES was requested to evaluate: 1. Whether a management strategy is precautionary if the TAC is constrained with a lower bound in the range of 20 000 tonnes to 40 000 tonnes and an upper bound in the range of 150 000 tonnes to 250 000 tonnes, or another range suggested by ICES. 2. Whether such a strategy would be precautionary if the TAC constraints referred to in paragraph 1 are overridden by a constraint on the maximum value of fishing mortality (Fcap), and whether the application of the Fcap would allow a precautionary strategy with a higher minimum TAC than if the Fcap was not applied. 3.

Whether a provision to override the minimum value of the TAC when the stock is forecast to be below some threshold value would allow a precautionary strategy with a higher minimum TAC than if the escape-clause was not included, and whether such a provision would provide any additional benefit to the inclusion of an Fcap as referred to in paragraph 2.

The alternative management procedures were tested in the framework of a management strategy evaluation (MSE) set up according to the assessment model SESAM adopted for Norway pout in the 2016 benchmark. One thousand simulations (replicates) were projected over 20 years for each of the different harvest control rules.

Each replicate begins in the 2018 TAC year which starts in quarter 4 of calendar year 2017. Each replicate randomly draws a true state of the system (starting population, age and quarterly fishing patterns and series of past recruitments) from the joint distribution estimated by the last stock assessment.

This is taken as the approach best reflecting the uncertainties in the SESAM assessment. An alternative reducing the uncertainty in the initial stock numbers, recruitment and exploitation pattern at the median estimate from the last assessment was also tested. The simulations were conditioned by a maximum realized level of fishing mortality the fishery can exert (assumed at 0.89; Fhistorical), which means that the full TAC will not be taken if the required F exceeds this value.

First the group tested whether the current ICES procedure for providing TAC advice for Norway Pout, based on an escapement strategy (the default method), was precautionary. Results showed that it is not precautionary (as tested with unconstrained levels of fishing mortality), because the probability of SSB falling below Blim is higher than 5%.

This is probably linked to cases of very high TAC and F when very high recruitments occur, in association with observation errors in the assessment. This called for modifying the default escapement strategy either by setting an upper F (Fcap) or including conditions on TACmin/ TACmax as explored here.

Concerning Request 1: The group tested HCR escapement strategies (as the default method) bounded by a combination of TACmin (at either 20, 30 or 40 kt) and TACmax (at 150 and 200 kt). Results show that these HCR were precautionary for TACmin at 20 kt for the two TACmax levels and for a TACmin at 30 kt when bounded by a TACmax of 150 kt.

They gave median and mean TACs (around 100–130 kt depending upon the rule) and realized catches around 110–115 kt, with TACs set at TACmin or at TACmax around 20–24% and 36–46% of the cases respectively. In these cases, Fhistorical was reached in 9–16% of the years, which makes the results sensitive to the assumption that the fishery will not exceed catches requiring F above Fhistorical.

Other combinations based on higher values of TACmin or TACmax led to unprecautionary outcomes. Concerning Request 2: The same combinations of TACmin and TACmax as for request 1 were explored with Fcap at either 0.3 or 0.4. Results showed that the inclusion of a Fcap increases the range of TACmin and TACmax combinations that are precautionary, reaching for Fcap up to a TACmin at 30 kt and a TACmax of 200 kt.

On average, TACs become considerably lower when Fcap is applied (ranging between 72 and 97 kt depending upon the combinations) and realized catches did not exceed 92 kt. In general, TAC increases with increasing Fcap. The gain in average TAC by increasing TACmin or TACmax is minimal, but the TAC constraints affect the probability of falling below Blim.

For these bounded rules, the probability of setting TAC at TACmin is very similar to the probability for HCRs without an Fcap, but the probability of reaching TACmax is considerably lower due to the application of the Fcap. The absolute changes in TAC between years are smaller with Fcap constraints as well, (in the order of 40 000 t) partly because of the lower TAC in general.

Applying Fcap makes the HCR more robust to violations of the assumption of an Fhistorical, as the probability of reaching Fhistorical becomes significantly lower than for the HCR without an Fcap.  The sensitivity of the performance of tested HCR to the alternative fittings of the stock recruitment relationship is minor, as shown for examples of rules with Fcap.  Concerning request 3, due to time limitation and little interest from stakeholders to override the TACmin, the group did not fully cover this request, but an exercise was made to find out if the TACmin of 40 000 t might become precautionary under an alternative configuration of the escapement policy.

An escapement strategy with a TACmin at 40 000 tonnes aiming at an escapement Biomass at 65 000 t instead of the current Blim (39 450 t) would become precautionary with combinations of Fcap in the range 0.3 to 0.4 and TACmax in the range 150 to 200 kt. TACmin would be set in around 48% of the years, which gives a median TAC slightly above TACmin.

Mean TAC for the three HCR is in the same order of size as for Request 2. The Special request also asked ICES to evaluate whether the results of the MSE would be significantly changed if the TAC year were defined as 1 November to 31 October rather than a calendar year. The latter TAC year is applied to the EU Member States fishing in EU waters, while Norway uses the calendar year (January–December).

Furthermore, ICES advice is produced based on a forecast from 1 October to 30 September, and ICES uses such forecast to advice management for the period 1 November- 31 October. The MSE adopted to answer the request follows the same practice. The WK has not compared the results of the MSE for the TAC year defined as 1 November to 31 October with those for a calendar year, as the latter would require a time shift in the assessment and forecast.

The report includes some considerations on the current practice for advice and the TAC year.