A dish with no fish: How DHA-free microalgae and linseed oil enhance n-3 LC-PUFA biosynthesis in rainbow trout (Oncorhynchus mykiss)

Fish are a source of essential n-3 polyunsaturated fatty acids for human nutrition. In fish diets, fish oil provides EPA and DHA but sustainable alternative lipid sources are needed. Vegetable oils can contain n-3 short-chain PUFA (SC-PUFA) yet no n-3 long-chain PUFA (LC-PUFA). However, freshwater fish are capable of partly biosynthesizing n-3 LC-PUFA if sufficient precursor n-3 fatty acids are available. The DHA-free microalgae Tetraselmis chui offers one of these precursors, namely stearidonic acid (SDA). To determine if T. chui could be a viable fatty acid source in fish diets and to gain more insight of the precursor-to-product relationship in the fatty acid biosynthesis, a feeding trial with rainbow trout was conducted to evaluate fatty acid metabolism and growth performance. Fish oil was partially replaced with microalgae in fishmeal-free diets ( 0 %, 33 %, 66 % and 100 %), and linseed oil was additionally supplemented to elevate the linolenic acid (ALA) content. Fish (89 g ± 1.1 g) were fed daily at 1.9% of their body weight for 12 weeks, followed by a digestibility trial. The microalgae inclusion decreased the diet digestibility and reduced the growth performance of the fish at 14 % algae inclusion. Nevertheless, dietary microalgae and linseed oil increased biosynthesized DHA. Moreover, ALA appeared to be a favored substrate for the biosynthesis over linoleic acid (LA). The fillets of fish fed the algae diets demonstrated suitable DHA levels and n-3 contents. T. chui demonstrated great potential as an alternative fatty acid source in fish nutrition, although lacking DHA. Combining microalgae with vegetable oils could be a promising approach for sustainable feed formulations.