Report of the Workshop to evaluate the TAC calculation for herring in IIIa and management plan for herring in the North Sea (WKHerTAC)

WKHerTAC evaluated the current (2008) LTMP for NSAS and the proposed TAC allocation strategy for herring in the IIIa following two joint requests from the European Commission (EC) and Norway. The evaluations of both requests were based on a stochastic medium-term simulation model. The model simulated the biological herring population and the behaviour of the fishing fleets and surveys, while the stock assessment estimates 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 were run with 1000 Monte Carlo realisations (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.
WKHerTAC considered that there were ambiguous points within the request and its background for herring in Area IIIa. The suggested management rule for IIIa could not be interpreted unambiguously. The results showed to be sensitive to different interpretations, which makes it vital that all the specific points in the management rule must be outlined very clearly. Consequently, ICES had to make a number of assumptions to make the provisions of the special request operable in a management strategy evaluation. 

The two stocks were modelled in parallel to account for the known mixing in the fisheries. The fraction of each stock in the catch in IIIa was treated as a random variable with a fixed mean independent of the abundance of the stocks. Recruitment was based on the recent low productivity regime for both stocks as estimated from 2003–2013 for NSAS and 2005–2013 for WBSS. The assessments were simulated by introducing variation in the population numbers and the exploitation pattern. Annual TACs by fleet were computed on the basis of short-term forecasts. The TAC setting procedures and allocation of catch potential to each of the five fleets considered followed from the management plan/rule and potential transfers from one area to another.
In total 15 HCR options were examined; six evaluating the long-term management strategy (LTMS) for herring in the North Sea and nine evaluating the consequences of the IIIa TAC setting procedure. For the LTMS the six scenarios evaluated a range of B_trigger values (1.0–1.5 million tonnes). All scenarios tested assumed a 15% constraint on TAC IAV, a 10% constraint on F (limiting departure from target F) and an interannual quota flexibility of ±10%.  For the IIIa TAC setting procedure, the basis of the scenarios tested mainly differed in the fraction of IIIa TAC transferred to the North Sea (0–50%).

All scenarios tested were considered to be precautionary for NSAS and WKHerTAC concluded that if the Btrigger for NSAS is to be revised, values at or above 1.0 million tonnes would be considered precautionary. For WBSS, WKHerTAC concluded that the IIIa management rule as it is currently stated is not precautionary. To make the management rule precautionary, several possible solutions could be investigated. The only option that has been tested is the % transfer which is an annually negotiated management decision. Other modifications to the rule such as reducing or capping the NSAS share of the C-fleet TAC would be likely to work in the same direction but were not evaluated. The evaluations have shown that the rule is not precautionary, throughout the entire simulation period, unless there is a transfer of at least around 10%. Additionally, the advice for the IIIa management rule is based on the B_trigger in the current NSAS LTMP of 1.5 million tonnes and if this value decreases (which would be precautionary for NSAS) then a higher than 10% transfer, or other measures, would be required to ensure that the IIIa management rule is precautionary. A lower Btrigger in the NSAS LTMP would lead to a higher TAC for the A-fleet, which in turn would result in a higher C-fleet TAC. Depending on the % transfer of TAC from the C-Fleet to the North Sea (or other modifications to the rule), it will most likely have implications for the utilisation of WBSS at or below F_MSY.