A mesocosm fish farming experiment and its implications for reducing nutrient load on a regional scale

This work presents results from a mesocosm fish farming experiment. The main aim of the work was to put the results from the experiment into a wider context dealing with an approach to get zero – or even – negative nutrient fluxes from fish cage farms on a regional scale. To quantify the nutrient loading of phosphorus and nitrogen from net cage aquaculture, a standard dynamic mass-balance model approach was used. A comparison was made between responses from two feeding scenarios with rainbow trout (Oncorhynchus mykiss). One set was given a commercial pelleted fish food the other food with fresh herring inclusion. We have shown that on a regional scale, a zero nutrient load situation may be achievable if the wild fish from the given region account for about 11% of the fish food. In the mesocosm experiment, we also tested different approaches to calculate the growth of the cultivated fish and one can conclude that there are no major differences in using the different sub-models for the interpretations to get zero emissions but different growth functions do give different dynamic responses for the fish growth. We have also tested if it is important to account for denitrification. Under the given conditions this is not the case. Critical testing of the modelling has been performed with uncertainty and sensitivity analyses and the major uncertainties were identified in the growth pattern of the fish and the mesocosm biomass nutrient uptake rate. We argue that these results imply that, from an eutrophication point of view, fish farming in open net cages can be viewed as an environmental sustainable industry, if the fish feed includes nutrients that originate from the surrounding waters.