Co-farming species from different trophic levels is commonly referred to as integrated multitrophic aquaculture (IMTA), where nutrients from fed species are consumed by extractive species. Nitrogen budgets can be used to evaluate the nitrogen remediation efficiencies of different farming systems, for example in recirculating aquaculture systems (RAS) . Nitrogen is of large importance since it affects both fish growth, health and welfare , water quality, and the surrounding environment. The nitrogen input to a system is typically through feed, which is the most climate impacting and costly factor in fish farming. Thus, nitrogen budgets can be used to evaluate both environmental and economic sustainability. In Sweden the production of farmed fish is dominated to almost 90% by rainbow trout while both Atlantic wolffish and macroalgae such as sea lettuce have been proposed as a promising cold-water species in aquaculture diversification, thus making them interesting for co-farming studies. The aim of this study was to evaluate the effects of co-farming rainbow trout and Atlantic wolffish with sea lettuce Ulva fenestrata on the performance of a closed seawater (SW) RAS through analysis of nitrogen budgets .
Rainbow trout and Atlantic wolffish were housed in a 18 m3 SW RAS for four consecutive periods , each lasting four weeks. During the first and third period, only fish were held in the system and during the second and fourth periods, fish were held together with Ulva in the RAS loop . A nitrogen balance and budget were calculated for the RAS for each period. N itrogen input to the system was through added feed. Nitrogen outputs included uptake by fish and Ulva , changes in dissolved inorganic nitrogen (DIN), DIN in wastewater and skimmer elute. The nitrogen uptake by Ulva was calculated from nitrogen in tissue content and biomass growth.
The nitrogen budgets varied between the periods and accounted for 47 – 68% of the added nitrogen. The exact total nitrogen held as fish biomass, in rainbow trout and Atlantic wolffish, varied over time between 87 - 91%. System DIN concentrations remained stable around 10 – 12 % of the total nitrogen. The Ulva held 3 - 4 percent of total system nitrogen at the end of the co-farming periods and assimilated between 7 and 11% of the total nitrogen input . The nitrogen content of Ulva increased with 29-35% dry weight (dw ) throughout both periods, thus indicating that the RAS water was beneficial for the macroalgae . Our results demonstrate that U lva is an efficient nitrogen assimilator and that co-farming with Ulva in RAS improves nitrogen utilization.