Stock assessment and sensitivity analysis of Scomber japonicus in 

the Northwest Pacific Ocean using Catch-MSY method

Abstract: Chub mackerel Scomber japonicus is an important economic and small pelagic species in the Northwest Pacific Ocean. Japanese fisheries initially exploited Scomber japonicus species in this area, and in recent years, the major countries harvesting this species include Japan, China, Russia and Korea. The annual catch of Scomber japonicus recorded in 2019 was about 64 364 tons in China, which accounted for 14.00% of the global production. With the global development of Scomber japonicus fisheries, the number of fishing vessels has been increasing, coupled with the impact of marine environment and climate change, Scomber japonicus resources fluctuate, and fishing production has undergone annual changes. Meanwhile, Scomber japonicus fishery is a typical datalimited fishery in the Northwest Pacific Ocean. It has the biological characteristics of a short life cycle and a long migration route, and the population is extremely sensitive to largescale climatic events and regional environmental changes in the Northwest Pacific Ocean. In 2015, Scomber japonicus has been listed among the priority fish species by North Pacific Fisheries Commission (NPFC). Accurate understanding of its stock status can provide strong guarantee for fishery management and sustainable development. The major global fisheries have been in datapoor/limit condition and the conventional stock assessments are available for stock based on accurate biological and catch data. As attaining high quality fishery statistics in a short period of time can be challenging, regional fisheries management organizations have determined that the datapoor approach is an appropriate research direction for the stock assessment of datalimited fisheries. CatchMSY method can temporarily replace conventional stock assessment models in making management decisions for a datalimited fishery, even when only catch data are available. Further, CatchMSY method requires less data compared to other methods. Consequently, this model may be a good choice for the MSY estimation of Scomber japonicus. CatchMSY model was used in this study to estimate MSY and stock status on the basis of catch data and life history information from the NPFC. CatchMSY model was based on the Schaefer production model, which was developed to calculate annual biomass, and the ranges of the initial and current depletion levels should be specified in CatchMSY model. In this study, we derived default ranges of relative biomass in the first and last years of the time series. For the first year, if the ratio of catch to the maximum catch was below 0.5, the range of B/K(γ1, γ2) was 0.50.9; otherwise, the range of B/K was 0.30.6. For the final year, the range of B/K(γ3, γ4) was 0.010.4 if the ratio of catch to maximum catch was below 0.5; otherwise, it was 0.30.7. What’s more, during the process, 16 scenarios, according to different prior distributions of the intrinsic rate of increase (r) and carrying capacity (K), were set for sensitivity analysis. Moreover, the influence of different catch time series and different process errors were taken into account. The results showed the following: 1) In standard scenario S1A, the estimated MSY of Scomber japonicus was 49.57×104 t (39.55×104 t to 60.58×104 t) and the value was 49.59×104 t (39.58×104 t to 60.62×104 t) in standard scenario S1B; 2) Compared with parameter r, K had a relatively narrow posterior distribution, lnK decreased gradually with the increase of ln r, and the value of MSY was positively correlated with the lower limit of r prior distribution; 3) When using the noninformative priors of parameter r, the posterior distribution of K had a broader range than using informative priors of r. When using the informative priors of parameter r, the lower limit r, the larger mean value of the posterior distribution of K, and the broader distribution of K values were used to maintain those yields. There was no significant difference of the posterior distribution of r when using noninformative priors and informative priors of K. 4) For r, the MSY increased gradually with an increasing lower limit of the prior distribution of r. The MSY was more discrete when the wider range prior distribution of r was given. For K, the upper limit of K only slightly influenced the distribution of MSY. There was a little fluctuation of MSY depending on the dataset, which meant that the time series of catch data had a limited impact on the estimation of MSY, whereas the results of the model were sensitive to the annual catch in the first and last years. The MSY value was sensitive to the time series of catch data, but its impact on the estimation of stock status was limited. 5) Given the uncertainty of the CatchMSY model, maintaining a management target between 50×104 t and 60×104 t was a better management regulation. This study shows that the CatchMSY model is a useful choice for estimating the MSY of datalimited species such as Scomber japonicus. Additionally, more types of fishery data of Scomber japonicus should be collected in future studies; subsequently, stock assessments of Scomber japonicus can be conducted using traditional stock assessment models, which may provide more reasonable suggestions for the development and management of Scomber japonicus.

Keywords: CatchMSY model; Scomber japonicas; sensitivity analysis; stock assessment; Northwest Pacific Ocea