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This paper presents a numerical study to assess the hydrodynamic effects of different levels of biofouling in fish cage aquaculture netting. The methodology adopted includes high resolution Large Eddy Simulation (LES) coupled to a regional model implemented within the framework of the Coastal and Regional Ocean Community Model (CROCO). The physical coherence of this approach has been previously verified and published by the authors. Three levels of biofouling were considered to describe a clean fish cage netting with no biofouling, medium biofouling and high biofouling growth levels. Clean and biofouled fish cage netting were described using a porous media model and the porous media coefficients were taken from the literature. A salmon farm located in the Estero Elefantes Channel in Patagonia (45°39′16.50 S 73°35′59.40 W) was used as study case. A change in the biofouling level in aquaculture fish cages netting can lead to drastic changes in the local velocity field. The high biofouling level produces the largest change in velocity direction and magnitude not only locally in near-cage areas, but also changing features in the overall fjord circulation. Velocity reduction in the shadow zone of fish cages reached values between 30% and 10% of the incident current for null and high biofouling levels, respectively. Simulations also showed dramatic effects of biofouling on flushing time. The predicted water availability of the salmon farm shows decreases of 27% and 36% considering medium and high levels of biofouling. This decrease in the available water is also related to a reduction in the amount of available oxygen and accumulation of waste products, affecting not only the health of farmed fish but also the environment. These and other environmental and productive implications are discussed. • The hydrodynamic effect of biofouled fish cage netting is addressed using coupled regional and LES ocean models. • A change in biofouling level can lead to drastic changes in the local velocity field. • Simulations show dramatic effects of biofouling on flushing time. • Water availability decreases of 27% and 36% considering medium and high levels of biofouling. [ABSTRACT FROM AUTHOR]