The structural and environmental challenges in the operation of marine fish cage structures, in particular in the open sea, can accurately be determined if the fluid dynamics in and around the whole system is studied. An important part of the system represents the porous net which encloses the fish. In this paper, a numerical model for the determination of the deformed shape of nets under consideration of hydrodynamic loads and elastic twines is elaborated. It is a quasi-static approach based on force equilibria at the knots and geometrical constraints. The missing time step restriction leads to an overall less costly computation. The fluid around the net is calculated using a two-phase CFD model. The focus of the research is on the calculation of fluid forces on the net and the analyses of the nets motion in current and waves.