Report of the Workshop on Herring Interim Advice on the Management Plan (WKHIAMP)
The WKHIAMP evaluated whether the existing management plan is precautionary un-der various settings for a TAC constraint, as variations to the current Long Term Man-agement Plan (LTMP), provided that expected recruitment is still low. All these variations to the LTMP come into play in the short term forecast where TACs are pro-posed for the A- and B-fleet given the harvest control rule and possible inter annual variation on TAC constraint.
The evaluation procedure was performed using a stochastic medium term simulation model. The model simulates the biological herring population and the behaviour of the fishing fleets and surveys, while the stock assessment is mimicked to estimate the stock status. Finally, the management advice and implementation are based on the adjusted management plan scenarios. In turn, management feeds back into the biological population and the fishery the year after. The simulations are run with 100 Monte Carlo realisa-tions (MCR) to obtain a broad range of possible outcomes given the variability in the input data. Stochasticity (randomness) was added to variables and parameters to ensure that biological variation, and the uncertainty in the historic perception of the stock was thus reflected.
The five HCR options that were examined were: 1) Current HCR; 2) Current HCR with-out constraint, 3) 0.2 – 0.3 HCR, 4) 50-50 HCR and 5) Current HCR without con-straint in 2012.
All options are in conformity with the precautionary approach, as the risk of SSB falling below Blim is always low under the assumed conditions.
The current HCR rule, with the 15% constraint, stands out as it allows only a slow increase in TAC from the low in 2011. It gives a similar or better stability than the other options, but it does that at the expense of a lower average yield, even in the medium term. Average F is 0.18.
The remaining four rules are similar in respect to average yields in the medium term. These four rules lead to an average F that is close to 0.25, which is regarded as an Fmsy, but they differ with respect to stability. The current HCR rule without constraint is the least stable, the 50-50 rule gives most stability, and the 0.2 – 0.3 rule is intermediate be-tween the two.
The stochastic simulation model has been designed to explicitly incorporate the natural and stock assessment variability as observed over the recent years which have led to considerable revisions in recruitment and spawning stock biomass. The evaluation presented here shows that the evaluated harvest control rules are all robust against this variability. However, the rules have not been evaluated against exceptional variations in biology which are beyond the variation observed in history, nor have the rules been tested for robustness under varying starting conditions in population size. These analyses, therefore, can be viewed as appropriate given the uncertainty in the current population size and they answer the request fully. However, they do not provide a full Management Strategy Evaluation.
Published under the auspices of the following ICES Steering Group or Committee