Influence of seasonality of bottom trawl surveys on assessment of stocks for bottom fish

Reliability of the fish stock assessments is affected by many factors, including seasonal patterns of fish distribution and inhomogeneity of distribution density. Results of two bottom trawl surveys are compared, which were conducted in the same area of Peter the Great Bay (Japan Sea) and in the same year 2013 but in different seasons, as spring and summer. The fish species stock assessments were mostly higher in summer, with exclusion of sculpins with insignificant difference between the seasons (except actively migrated graypurple sculpin). The observed difference was reasoned by seasonal bathymetric migrations. Therefore, an optimal for trawl surveys season can be determined taking into account the bathymetric migration of bottom species. In spring, the fish concentrated mainly at the depths more than 300 m, with the maximum density of 24.86 t/km² in the range of 400–500 m, where walleye pollock and blackfin flounder prevailed. In summer, the highest concentration was found in the range of 20–50 m but its value was much lower, 14.5 t/km². No more than 10 % of fish biomass remained in the deep-water areas with the depth from 200 to 700 m. The fish concentration in dense but small-sized aggregations increases inhomogeneity of the biomass mosaic distribution and reduces accuracy of their stock assessments. That’s why the summer season with relatively even distribution of bottom fish is the optimal time for bottom trawl surveys.