Essential fatty acids for fish need to be derived by their diets and fish oil is still a main source for EPA and DHA. With increasing inclusion of vegetable oils in fish diets, the n-3 long chain polyunsaturated fatty acid (LC-PUFA) content in fish is decreasing. Microalgae have already been evaluated in several studies for their application as a feed ingredient in aquaculture and due to their high contents of PUFA, they are considered as a valuable lipid source in fish feed. As the fatty acid profiles differ between species, their inclusion in diets should be balanced to the requirements of the fish. Especially microalgae with higher contents of stearidonic acid (SDA) could play an important role in enhancing the n-3 LC-PUFA biosynthesis in fish. In this trial the use of microalgae as n-3 fatty acid source was tested. The influence of EPA and DHA in fish oil free diets with elevated SDA contents was further evaluated, with a focus on the fatty acid biosynthesis in rainbow trout.
The microalgae species Isochrysis galbana (14.7 % ALA, 10.1 % SDA, 1.3 % EPA, 9 % DHA of total fatty acids), Tetraselmis chui (14.7 % ALA, 6.2 % SDA, 6.8 % EPA) and Schizochytrium limacinum (0.3 % ALA, 0.4 % SDA, 0.7 % EPA, 55.8 % DHA) were used to formulate four isonitrogenous, isolipidic and isocaloric diets with varying fatty acid profiles. 180 rainbow trout were held in a recirculating aquaculture system in triplicates of trial diets with 15 fish each. They were fed daily until apparent satiation for 56 days. For digestibility analysis the fish were fed for two more weeks and were manually striped. The fatty acid balance method was used to evaluate the bioconversion of fatty acids in the fish.
The combination of I. galbana and T. chui resulted in highest growth performance despite lower overall diet digestibility, which was compensated by higher feed intake. There was no dietary influence on the whole body nutrient composition, but the different dietary fatty acid profiles were mirrored in the fish with higher dietary DHA contents resulting in higher DHA contents in the fish. However, DHA appearance showed no product inhibition on fatty acid biosynthesis. Microalgae can be used as an efficient source for fatty acids in fish diets considering their different fatty acid profiles. Dietary DHA contents did apparently not influence the biosynthesis of this fatty acid in rainbow trout but higher EPA contents resulted in increased DHA production.