Biofouling by benthic organisms (barnacles, mussels) must be considered for tidal turbine operation and main- tenance because it modifies hydrodynamics (drag and resistance) and could be detrimental to the turbine perfor- mance. We investigate vortex modifications downstream of a tidal turbine due to biofouling using numerical modeling. Firstly, 2D flow downstream of a clean Darrieus vertical axis tidal turbine is simulated using a dynamic mesh for different tip speed ratio. Results agree former studies. Simulations are very sensitive to turbulence modelling. To ensure an acceptable computing time, only Large Eddy Simulation (LES) and Reynolds Averaged Navier- Stokes (RANS) models are used. Secondly, an airfoil with barnacles is modelled in two dimensions for various fouling heights and spacings. Then barnacles and mussels with various characteristics are fixed on blades. Vorticity fields are strongly changed by organism shapes. Mussels size has little impact on vorticity patterns. A few mussels could have stronger impact than a fully colonization. A 3D simulation is performed with a shape of barnacles from new In-situ measurements. Finally a colonized tidal turbine is simulated. Vortices around the tidal turbine are clearly modified.