A fatty fate: How different dietary microalgae affect the fatty acid metabolism and deposition in rainbow trout (Oncorhynchus mykiss)

Fish oil continues to be the primary source of EPA and DHA in aquaculture diets. As alternative sources, microalgae have been studied and shown to be a valuable lipid source in fish feed due to their high PUFA contents. Some freshwater fish are capable of biosynthesizing n-3 LC-PUFA if sufficient precursor fatty acids are available. Stearidonic acid (SDA) can be used as a substrate, bypassing the first biosynthetic step based on linolenic acid (ALA). This study examined the utilization of SDA-rich microalgae as a source of n-3 fatty acids for rainbow trout in substitution for fish oil. In addition, the influence of dietary EPA and DHA supplied by microalgae on the fatty acid biosynthesis was investigated in fish oil-free diets with elevated SDA content. The microalgae species Isochrysis galbana, Tetraselmis chui and Schizochytrium limacinum were used to formulate isonitrogenous, isolipidic and isocaloric diets with varying fatty acid profiles and n-3 fatty acid ratios. Rainbow trout ($\sim$130 g) were fed daily until apparent satiation for 56 days, followed by a digestibility analysis. The inclusion of I. galbana and T. chui resulted in a higher feed intake but increased the feed conversion ratios. Fish fed a combination of both microalgae demonstrated the highest growth rates. However, diet digestibility was reduced by microalgae inclusion. The different dietary fatty acids of the microalgae were displayed in the fish with efficient utilization of SDA and other n-3 fatty acids. Interestingly, the presence of dietary DHA did not appear to inhibit DHA biosynthesis but to increase DHA deposition. The microalgae utilized in this study were a suitable source for fatty acids in diets for rainbow trout and have the potential to substitute fish oil.