Co-farming rainbow trout (Oncorhynchus mykiss) and sea lettuce (Ulva fenestrata) increased macroalgae protein content and positively affected fish welfare

Integrated multi-trophic aquaculture (IMTA), with fed and extractive species in recirculating aquaculture systems (RAS), has been identified as a promising and sustainable farming system with the potential to increase nutrient utilization. Additionally, co-farming has been shown to improve the nutrient content, growth of the farmed organisms, and water quality parameters in the RAS. However, there is a knowledge gap on how co-farming can interact with health and welfare of the co-farmed species. In this study it was shown that co-farming rainbow trout, Oncorhynchus mykiss , with sea lettuce, Ulva fenestrata , in a salt-water RAS had beneficial effects on both organisms and improved the water quality. Throughout the experiment, the sea lettuce grew, and its crude protein content increased by 25–30 % during the four week co-farming periods. Within the RAS, sea lettuce also contributed to locally increasing the pH. However, no systemwide effects were observed on pH or dissolved nitrogen concentrations, suggesting that a larger macroalgae biomass should be used in relation to the total water volume and fish biomass. It was observed within the co-farming experiment that rainbow trout showed an improvement in observational welfare indicators such as less snout wounds, along with reduced plasma K + concentrations. There were no negative effects on the remaining physiological and welfare indicators. This study demonstrates that co-farming sea lettuce with rainbow trout can benefit the RAS and sea lettuce without compromising fish health and welfare. • Sea lettuce increased the pH of RAS water when co-farmed with rainbow trout • Co-farming with rainbow trout increased protein content of sea lettuce • Co-farming with sea lettuce improved rainbow trout Observational Welfare Indicators