Assessment of the stock status for greenland halibut (<i>Reinhardtius hippoglossoides matsuurae</i>) in the Okhotsk Sea

Abstract

State of the greenland halibut stock in the Sea of Okhotsk fishing zone is evaluated as overfished with a high probability of 97.5 % by the index of fishable biomass, and the overfishing continued in 2021. This conclusion is based on results of double filtering the posterior parameter estimates in the state-space generalized surplus production model JABBA (Just Another Bayesian Biomass Assessment), with additional tuning of the new model taking into account these results. The overfishing was not caused by significant changes of environmental parameters (average SST and EOF modes of SST were examined), but its main reason was the overestimated target harvest rate established for the Far East of Russia as 10 % of the total biomass or a half of natural annual mortality (20 %). This incorrect value of the rate was based on incorrect determination of age structure and terminal age for the halibut by the fish scale method. The new approach of the stock status evaluation with JABBA model is independent on the age data. It includes the additional filtering of the JABBA model result, its refinement with stringent tuning using the algorithm of No-U-Turn sampler, and checking additional parameters on hyperstability or hypersensitivity. The new reference point of the target harvest rate is between 2.5 and 5.4 % (95 % credible interval) of the total biomass, with a median of about 3.75 %. This new value is slightly higher than that one used by ICES for greenland halibut in the northeastern Atlantic (3.5 % for fishable biomass, its reduction to 2.5 % is discussed), but these values cannot be compared because of different habitat conditions and probably different growth rate of greenland halibut in the Okhotsk Sea and the Atlantic. Researches on the optimal level of greenland halibut exploitation should be continued; anyway, the new reference point of the target harvest rate has to be implemented immediately and to be used at least until correction of the age-length key for greenland halibut in the Okhotsk Sea. The input data and Stan code of the new model are presented in the annexes.