AEI prepress abstract  -  DOI: https://doi.org/10.3354/aei00259

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

Marie Maar*, Janus Larsen, Karsten Dahl, Bo Riemann

*Email: mam@bios.au.dk

ABSTRACT: Aquaculture production of rainbow trout is planned to increase offshore in the inner Danish waters and the expected release to the environment is 100 t-nitrogen (N) and 12.8 t-phosphorus (P) per farm annually. We applied 3D modelling to estimate spatial- and temporal effects of two potential future offshore fish farms on water quality, sediment fluxes and nutrient transports in the Samsø Belt area. The model results showed that water quality (mainly dissolved inorganic nitrogen and primary production) was affected in open waters during the production period leaving no traces one month 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 of water quality. However, the additional nutrient enrichment counteracts the required nutrient reductions from other sources with the aim to improve the ecological status. Likewise, far-field nutrient transports was 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.