From the point of view of mechanics, trawls are considered as extremely flexible and partly extensible rope and net structures which are exposed to flow. Form and loads of such gears mainly depend on the corresponding velocity of inflow and also on the so-called rigging elements that are required for the horizontal and vertical spreading of the fishing gear. At the same time the fishing gear is acting on the surrounding fluid. These reactions can on the one hand lead to unsteady states in the fishing gear. On the other hand changes of pressure and velocity can be detected by the fish and can possibly influence the selectivity of the fishing gear. This lecture is focused on the presentation of special numerical and experimental methods both for calculating large net systems and for analysing the reactions of the structure to the fluid.