Modelling the environmental impacts of future offshore fish farms in the inner Danish waters

There are plans to increase offshore aquaculture production of rainbow trout in the inner Danish waters, with an expected annual release into the environment of 100 t N and 12.8 t P per farm. We applied 3D modelling to estimate the spatial and temporal effects of 2 hypothetical offshore fish farms on water quality, sediment fluxes and nutrient transports in the Samso Belt area. The model results showed that water quality (mainly dissolved inorganic N and primary production) was affected in open waters during the production period, leaving no traces 1 mo after the production period. Changes in bottom water oxygen levels and Secchi depths were not considered important for the system. Sediment organic content and nutrient fluxes increased from reference conditions locally below the fish cages. The sediment did not recover from the impact between the production periods, especially at the site with lowest resuspension. There was a gross transport of nutrients into the coastal water areas corresponding to 21% N and 16% P of the farm inputs, but this only resulted in minor changes in water quality. However, the additional nutrients from the fish farms counteract implemented nutrient reductions from other sources which aim to improve the ecological status, and the excessive nutrients were shown to reach several protected Natura 2000 sites. Future implementation of fish farms in the area requires detailed spatial planning, optimisation of farm design and nutrient compensation by marine measures, e.g. blue mussel farming.