A new study reported by the Science for Environment Policy news service of the European Commission, indicates that a salmon farming system incorporating seaweed and sea urchins could reduce nitrogen releases to the environment by 45%. The study by Lamprianidou, F., et al. was published in the Journal of Estuarine, Coastal and Shelf Science and is based on a model for designing sustainable fish farming systems and calculating their potential to recycle waste. The study used data from Scottish farms to simulate a sea salmon farming system that combined growth models for Atlantic salmon, sea urchins and a seaweed (Ulva lactuca). The hypothetical system was composed of nine 90-metre wide salmon cages capable of producing 1000 tonnes of salmon and requiring 65 tonnes of nitrogen — in fish feed form — every 18 months.
The model system should produce 342 tonnes of seaweed and 20 tonnes of sea urchins every 18 months, in addition to the 1000 tonne target for salmon. The model predicts that only 22 tonnes out of the 40 tonnes of nitrogen excreted by the fish are released into the environment, thanks to the its recovery by seaweed and sea urchins. By contrast, a non-IMTA, salmon-only system would release all 40 tonnes (45% more nitrogen) to the environment. In such a farm, measures would be put in place to manage the nitrogen build-up in the form of ammonia, in order to prevent stress and disease in fish2 and to maintain the quality of fish products.