Standardization of CPUE for walleye pollock in the Okhotsk Sea with inclusion of some environmental factors

Abstract

Catch of walleye pollock by Russia is the highest in the northern Okhotsk Sea where on average 0.94 million metric tons were caught annually in the period between 1962 and 2017, or around 24 % of the total yield of Russian fishery. The total stock and spawning stock of pollock grow there since 2002, though the catch per unit effort (CPUE) has significantly decreased in the beginning of 2018 despite expected high levels of both total and spawning stocks. The sea surface temperature, ice cover and storms frequency were examined as possible reasons of low fishing efficiency in 2018. For this purpose, the generalized linear models (GLM) and generalized additive models (GAM) of catch dynamics are compared. GAM with addition of temperature and storms factors has the lowest Schwarz’s Bayesian criterion and the highest explained deviance (61.6 %). Efficiency of fishing gears has nonlinear relationship with the towing time. CPUE has hypersensitivity to the stock biomass presented as the power dependence (γ = 0.94, r = 0.923). Standardized CPUE is recommended for using in the final GAM for the pollock stock assessment in the northern Okhotsk Sea, hypersensitivity of CPUE should be estimated and corrected if necessary.