Introduction
Aquaculture is a rising sector and to maintain increasing fish production, an equal increase in feed production is necessary (Tacon, 2020). Especially fish meal and oil play an important role as protein and omega-3 fatty acid source in fish nutrition (Tocher, 2015). However, regarding sustainability and limiting wild fish stocks, alternative feed materials are currently of high interest. EPA (20:5 n-3) and DHA (22:6 n-3) are known as important long-chain polyunsaturated fatty acids (LC-PUFA) derived from fish in human nutrition as well as important fatty acids for the fish health itself. As fish are not able to produce omega-3 PUFA de novo, these fatty acids must be supplied by their diet. Despite to most saltwater fish species, rainbow trout are capable of biosynthesising the LC-PUFA EPA and DHA if enough ALA (18:3 n-3) as precursor is provided (Henderson and Tocher, 1987). Microalgae as primary producers in the marine food chain bring a broad offer of different nutrients and fatty acid profiles. The microalgae Tetraselmis chui showed a good potential as fatty acid source (Simon et al., 2024). It contains no DHA but low amounts of EPA and additionally offers ALA and SDA (18:4 n-3), an intermediate of the n-3 biosynthesis. In this study different n-3 PUFA profiles with increasing algae inclusion were evaluated for their effect on this biosynthetic pathway.
Material and Methods
To test if T.chui could be an suitable fatty acid source in fish diets, a feeding trial with rainbow trout was conducted to evaluate growth performance and fatty acid utilisation of the fish. Fish oil was partially (0 %, 33 %, 66 %, 100 %) substituted by the microalgae resulting in decreasing DHA concentrations in the diets. Additionally, two diets (fish oil 0 % and 100 %) were enriched with ALA by adding linseed oil. Triplicates of groups of 20 fish per diet were fed daily with 1.9 % of their body weight for 12 weeks following a 2-week faecal collection period to determine digestibility.
Results and Discussion
The algae diets resulted only slightly in lower growth performance of the fish, although showing a lower digestibility in accordance with other studies of microalgae (Cardinaletti et al., 2018; Sarker et al., 2020). No significant effect was observed regarding health parameters of the fish as well as no effect in the whole-body nutrient composition. However, with increasing algae inclusion the DHA contents in the fish are decreasing as fatty acid composition of the fish mostly mirrors diet composition. But following the n-3 pathway an increasing proportion of biosynthesised DHA (Fig. 1, circles) can be seen, underlining the ability of rainbow trout to produce this fatty acid from precursors (Sargent et al., 2002). The addition of ALA did not influence the growth performance but increased the DHA production (Fig. 1, rectangles).
Conclusion
T.chui showed great potential as alternative fatty acid source in fish nutrition although lacking in DHA. Rainbow trout could compensate this lack by DHA biosynthesis and the increase of dietary ALA additionally promoted the DHA production. Understanding the physiological fatty acid pathways in the fish is essential for future feed formulations as sustainable use of important fatty acids is needed.
References
Cardinaletti, G., Messina, M., Bruno, M., Tulli, F., Poli, B.M., Giorgi, G., Chini-Zittelli, G., Tredici, M., Tibaldi, E., 2018. Effects of graded levels of a blend of Tisochrysis lutea and Tetraselmis suecica dried biomass on growth and muscle tissue composition of European sea bass (Dicentrarchus labrax) fed diets low in fish meal and oil. Aquaculture 485, 173–182. doi:10.1016/j.aquaculture.2017.11.049.
Henderson RJ, Tocher DR. 1987.The Lipid composition and biochemistry of freshwater fish. Progress in Lipid Research;26:281–347.
Sargent, J.R., Tocher, D.R., Bell, J.G., 2002. The Lipids: Fish Nutrition. Elsevier Science.
Sarker, P.K., Kapuscinski, A.R., Vandenberg, G.W., Proulx, E., Sitek, A.J., 2020. Towards sustainable and ocean-friendly aquafeeds: Evaluating a fish free feed for rainbow trout (Oncorhynchus mykiss) using three marine microalgae species. Elementa: Science of the Anthropocene 8. doi:10.1525/elementa.404.
Simon, A., Lippemeier, S., Mueller, J., Schlachter, M., Kaiser, F., Schulz, C., 2024. A question of digestion: How microalgae species affects lipid and fatty acid digestibility in rainbow trout (Oncorhynchus mykiss). Aquaculture 593, 741311. doi:10.1016/j.aquaculture.2024.741311.
Tacon AGJ. 2020. Trends in Global Aquaculture and Aquafeed Production: 2000–2017. Reviews in Fisheries Science & Aquaculture;28(1):43–56. https://doi.org10.1080/23308249.2019.1649634.
Tocher DR. 2015.Omega-3 long-chain polyunsaturated fatty acids and aquaculture in perspective. Aquaculture;449:94–107. https://doi.org10.1016/j.aquaculture.2015.01.010.