Simple relationships expressing the dependence of relative sampling error in a biomass estimate (from a survey) on the statistical characteristics of the fish concentration density field and on parameters of the survey itself, have been derived with the help of mathematical statistics and methods of calculus of probabilities. The biomass estimate is determined as the product of the average density for the region under examination and the area of this region. For a hydroacoustic survey, the anisotropy parameters correlation radius along transects and coefficient of variation serve as field characteristics on which the error depends. The direction of the survey with respect to the axis of the correlation ellipse and the frequency of transects (sections) serve as survey parameters. The relationships described here can be used in practice for both a posteriori estimation of the error made when calculating the biomass, and for survey planning on the basis of a priori estimates of the statistical characteristics of the concentration density field. These might provide a basis for the operative control of surveys.