Workshop on the evaluation of northern Norwegian coastal cod harvest control rules (WKNCCHCR)
The workshop on the evaluation of northern Norwegian coastal cod harvest control rules (WKNCCHCR 2022) was held to evaluate proposed HCRs and to examine the basis of the Blim for the coastal cod north stock. The stock has moved from category 3 to category 1 with a SAM assessment as a result of the benchmark WKBARFAR in 2021, and there was therefore a desire to produce HCR(s) for the stock based on the new assessment. In addition, concerns had been raised over the estimation of Blim at WKBARFAR. The meeting thus covered both an examination of the model and data relevant to Blim, as well as evaluations of various HCRs.
While investigating Blim it was discovered that the estimate of Blim was highly sensitive to small changes in model or data, or even to the choice to consider recruitment-at-age 2 vs. age 3. These changes moved the Blim estimate in a range between lowest and highest observed stock size and various points in between. Over the observed range of SSB recruitment had never completely collapsed but it was difficult to distinguish to what extent, if any, recruitment had been impaired. This was taken to imply that the signal in the recruitment was weak both due to noise in the data but also due to the nature of the stock as a complex of interacting substocks (various fjord substocks plus a separation-by-distance stock along the coast), meaning that there is no one single Blim value associated with impaired recruitment for all of the substocks. As a result, it was concluded that no reliable Blim estimate could be produced for this stock.
As part of the work on Blim, the meeting identified that the acoustic survey has poor cohort consistency, particularly in the most recent portion (the index is split between 2002 and 2003 due to a change in methodology). The decision was therefore taken to change to using the acoustic index as a biomass index instead of an age-structured index. This change has only a minor impact on the SSB estimates.
The request required an examination of HCRs with a combination of Btrigger and both FMSY- and Fp0.5 (Fpa)-based fishing advice. However, given the absence of a reliable Blim level which the stock should be maintained above, it was considered that none of these HCRs could be considered precautionary, and therefore could not be used as the basis for ICES advice.
To arrive at an HCR which could be evaluated as precautionary and form a basis for potential ICES advice, the meeting proposed an HCR based on F0.1. In addition to the theoretical basis for F0.1 as a fishing level that should drive the stock to safe and productive levels, the F0.1 estimate was rather stable to the range of data and model options tested at WKNCCHCR and at similar previous fishing pressures the stock had increased. Further, in all cases, F0.1 was estimated as being below all potential values of Fp0.5 (either with or without a Btrigger point and taking account of the uncertainty around Blim). The HCR with Ftarget = F0.1 was therefore evaluated as precautionary within the observed range of SSB in the part of the time-series considered in the stock–recruit simulations (2003–2020). This lower bound on the HCR was named SSB_lowerbound.
The meeting also examined the Fbar range used for reporting F and concluded that the Fbar range should be expanded from the current F4–7 to F4–8.
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