Numerical studies on the hydrodynamic effects of a salmon farm in an idealized environment

•A LES model with a momentum sink simulated the impact of net cages on hydrodynamics.•The model predicts the wake formation downstream of the salmon farm.•Wake properties depend on net drag coefficient and freestream velocity.•The wake is characterized by a low velocity core surrounded by a high diffusion zone.•Passive tracer released inside farm was not advected for low drag nets.

A Large Eddy Simulation (LES) model was used to describe the hydrodynamic effects of a salmon farm on an incident current with constant and semidiurnal variability. In particular, we describe the formation of a zone of low velocity (wake) downstream of the farm and the advection of a passive tracer released inside the farm. The mesh of salmon farm net cages was described using a porous jump approximation, validated with experimental data available in the literature. A set of simulations were run to analyze the effects of the mesh drag coefficient and the current intensity on wake formation and current dynamics. The results show that the extension of the wake depends directly on current intensity, where larger extensions are associated with more intense currents; whereas the drop in velocity within the wake appears to be solely a function of the mesh drag coefficient, where meshes with higher drag coefficients generate higher velocity drops. We also find high turbulent diffusion in the contour area of the wake, attributable to a high strain rate in that region. Moreover, given semidiurnal current variability, we find the formation of a positive wake (high velocity area) at the beginning of flow direction change during each semidiurnal period. Additionally, we analyze the advection of a passive tracer initially released inside the salmon farm, which indicates that, in fish farms with meshes of high drag coefficient, the tracer is almost entirely advected away from the vicinity of the farm after the third simulation day; while for low drag meshes, the tracer is retained around the salmon farm. We discuss the potential application of this model for analyzing the transport of organic and inorganic components, such as organic matter, nutrients, parasites and antibiotics, and the impacts on biologically relevant pools of non-conservative elements, such as oxygen.