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

Abstract

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 tP per farm. We applied 3D modelling to estimate the spatial and temporal effects of 2 hypotheticaloffshore fish farms on water quality, sediment fluxes and nutrient transports in the Samsø Beltarea. 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 afterthe production period. Changes in bottom water oxygen levels and Secchi depths were not consideredimportant for the system. Sediment organic content and nutrient fluxes increased fromreference conditions locally below the fish cages. The sediment did not recover from the impactbetween the production periods, especially at the site with lowest resuspension. There was a grosstransport of nutrients into the coastal water areas corresponding to 21% N and 16% P of the farminputs, but this only resulted in minor changes in water quality. However, the additional nutrientsfrom the fish farms counteract implemented nutrient reductions from other sources which aim toimprove the ecological status, and the excessive nutrients were shown to reach several protectedNatura 2000 sites. Future implementation of fish farms in the area requires detailed spatialplanning, optimisation of farm design and nutrient compensation by marine measures, e.g. bluemussel farming