Credibility in spatial data from Fishery Monitoring System pertaining to catches of demersal fish 1) We attempted to verify reliability of spatial data from Fishery Monitoring System pertaining to catches of demersal fish. 2) Movements and catches of a vessel which carried out bottom trawling were analysed for the period from 2 to 20 February 2018. 3) It was shown that official catch in West Bering Sea zone (WBZ) exceeded the official catch in Karaginskaya subzone (KS) 125 times over for pacific cod and 970 times over for flounders. 4) Based on the assumption about equal distribution of biomass densities for pacific cod and flounders in KS and adjoining area of WBZ, we recalculated their catches and revealed that 146 t of pacific cod and 152 t flounders (two thirds of total catch for each object) presumably were caught in KS but were written in WBZ.