The paper presents calculations of water flow velocity through the codend under the assumption that the flow is uniform and equal on its entire surface. The calculations were made for different ratios of cod end length to its diameter, mesh bar length to mesh bar diameter, and for various mesh opening coefficients. The results obtained were analysed with regard to the impact of flow velocity on hydrodynamic forces opening the codend. These forces, different for different cod end constructions, have great impact on selection properties. The greater velocity of water outflow is effecting the wider opening of the codend meshes and a greater probability of undersized - juvenile fish escaping.
An idea of a new setup of meshes in the codend, changing the effect of operation of longitudinal forces (hydromechanical drag forces) was presented. Closing forces become mesh opening forces, which should ensure greater mesh opening coefficient and better selectivity of the codend of the trawl.