Farmed Sea Lettuce (Ulva fenestrata) as a Promising Ingredient for Rainbow Trout (Oncorhynchus mykiss)

Introducing novel and sustainable feed ingredients is essential to improve the environmental impact of fish aquaculture. Some species of seaweed, such as the green alga sea lettuce (Ulva fenestrata), constitute a promising novel ingredient due to their suitable amino acid profile and favourable growth traits. However, the effects of farmed Ulva in feed for cold-water salmonids have not yet been studied. Here we show that rainbow trout (Oncorhynchus mykiss) growth was improved by including Ulva meal in the feed while maintaining intestinal health and fish welfare. The study lasted for 12 weeks, and experimental diets included a control feed, and two feeds with 3% and 15% Ulva meal, respectively. The growth rate was highest for fish given the 15% Ulva diet during the first 6 weeks and the 3% Ulva diet for the last 6 weeks. This was likely due to an increased feed intake (FI) and improved palatability resulting from the inclusion of Ulva. Analysis of intestinal health parameters, primary barrier function and histomorphology, revealed no impaired barrier or inflammatory responses to any of the diets. Similarly, other primary and secondary stress indicators including plasma cortisol levels and oxidative stress markers in the liver and muscle tissues, were unaffected by dietary treatments. Interestingly, the fillet colour became more yellow with increased Ulva inclusion, which could affect the acceptance of the product to customers. Nevertheless, the results demonstrate that sea-farmed Ulva can contribute to improving the sustainability of future aquafeeds for rainbow trout.