Integrated multi-trophic aquaculture (combined production of fish, mussels and seaweed)

The Danish marine aquaculture has, despite the huge potential, only been slowly increasing the last 25 years because of the imposed limits to the nitrogen (N) released to the environment. Mussels, seaweed and other organisms have been successfully tested as biofilters in integrated multi-trophic aquaculture (IMTA) worldwide, where nutrients emissions (especially N) from e.g. fish production are assimilated and removed by valuable biomasses (crops; Fig.1).

This IMTA production unit, and even spatial decoupling of the biofilter organisms from the fish, have been recognized by the Danish national authorities in off-shore fish farming. The bioremediation potential and yield of the “new” crop, seaweed (sugarkelp, Saccharina latissima) was monitored in a commercial off-shore IMTA system year round at Hjarnø Havbrug fish farm near Horsens.

Furthermore, the year-round protein, amino acid, fatty acid, mineral and vitamin content and profiles were monitored to evaluate the nutritional value and harvest time of the seaweed biomass. Sugarkelp showed to be efficient for bioremediation of nitrogen, with environmental and potentially economic benefits (e.g. waste water management and for production of valuable biomass).

The seaweed protein content varied throughout the experimental period with the highest values recorded in November (14-20% of dry weight) and the lowest values recorded in May-July (2.8-6.7%). The lowest lipid content was observed in July, while the highest values were observed in November (approx. 4% of dw), with EPA (20:5(n-3)) and DHA (22:6(n-3)) accounting for 11.3-14.4% and 2.5-4.6% of total fatty acids, respectively.

This “new” Danish aquatic crop has potential applications. The harvest time should be settled around May for human consumption and September in order to achieve maximum biofiltration efficiency with harvested biomass for feed utilization. This considering both biology/life cycle, biofouling and yield, which may compromise with seasons with higher nutritional value of the produced biomass.