Integrated marine cultivation of Gracilaria chilensis (Gracilariales, Rhodophyta) and salmon cages for reduced environmental impact and increased economic output

Fish farms produce large amounts of wastes, including dissolved inorganic phosphorus and nitrogen. To exploit these nutrients as a resource input, and at the same time reduce the risk for eutrophication of the environment, Gracilaria chilensis on ropes was co-cultivated with a salmon cage farm in southern Chile during two summer month. Gracilaria cultivated at 10 m distance from the cages had up to 40% higher growth rate (specific growth rate; 7% day −1) compared to growth at 150 m and 1 km distance. The algae nutrient content was also higher close to the cages (1.9–2.1 mmol N g −1 dw −1 and 0.28–0.34 mmol P g −1 dw −1) compared to the other stations. Yield of agar per unit biomass varied between 17–23% of dry weight and was lowest close to the farm, but due to higher growth rate, the accumulated agar production was highest close to the fish cages. The degree of epiphytes and bryozoa coverage was overall low. An extrapolation of the results showed that a 1 ha cultivation of the algae, close to the fish cages, had the potential to remove at least 5% of dissolved inorganic nitrogen released from the fish farm and 27% of released dissolved phosphorous. Such a cultivation would give an annual harvest of 34 dry t of Gracilaria, worth 34000 US$. We conclude that both economical and environmental advantages can be achieved by integrating the cultivation of algae with fish farming in open sea systems. However, further studies focusing on full scale cultivation and functions during different seasons are needed.