Unsustainable fishing practices have placed a heavy emphasis on aquaculture to meet the global shortfalls in the supply of fish and seafood, which are commonly accepted as the primary source of health-promoting essential omega-3 (n-3 highly unsaturated fatty acids). However, dietary fish oil is required for the production of omega-3-rich farmed fish and this commodity, in a vicious circle, is at present derived solely from wild fisheries. Decreasing global availability coupled with the highly variable price of this resource has forced the aquaculture industry to investigate the possibilities of alternative dietary lipid sources. This review attempts to compile all principal information available regarding the effects of fish oil replacement for the diets of farmed finfish, analysing the findings using a comparative approach among different cultured fish species. The review initially focuses on the present situation with regard to the production, availability and main nutritional characteristics of fish oil and the principal alternative lipid sources (such as vegetable oils and animal fats). Following this, the effects of fish oil replacement in finfish nutrition on feed quality, fish performance, feed efficiency, fish lipid metabolism, final eating quality and related economic aspects are presented and discussed.

Fish oil replacement in finfish nutrition

Essential fatty acids (EFA) remain one of the least well-understood and enigmatic nutrients in aquaculture nutrition. Of all dietary nutrients none has a greater direct impact on the composition of its consumer. Their importance stems not only to their impact on animal growth, but also to factors such as reproduction, immunity and product quality. Docosahexaenoic acid (DHA; 22:6n-3) has consistently been shown to provide the greatest EFA value to most species. However, the nutritional value of eicosapentaenoic (EPA; 20:5n-3) and arachidonic (ARA; 20:4n-6) acids has also been significantly greater than that exhibited by linolenic (LNA; 18:3n-3) and linoleic (LOA; 18:2n-6) acids. All five fatty acids have been shown to provide EFA value to most aquaculture species, although the optimal dietary inclusion levels and balance among the fatty-acid classes (n-3 and n-6) and fatty-acid chain lengths (18-C, 20-C or 22-C) vary among species. Environmental origin (freshwater, estuarine or marine) appears to be a primary factor influencing the difference in EFA requirements. The role that EFA play in osmoregulation clearly shows how these nutrients affect animals from different aquatic environments. The influence of EFA on growth also appears to be greatest in larval fish and crustaceans, possibly because of their reduced ability to digest and absorb lipids, but also because of a proportionally higher demand for EFA in the development of, in particular, neural tissues. Despite an abundance of research since the 1970s on the EFA requirements of aquaculture species there remains a need to better define the EFA requirements of most aquaculture species. Of all major aquaculture species only the penaeid shrimp has a comprehensively documented assessment of its nutritional requirements for EFA. The nutritional requirements for EFA in most fish species have not been comprehensively studied and those species that were fully examined in the 1970s and 1980s now need to be reassessed in light of recent changes to the use of high-nutrient-density diets that were not routinely used in either practice or research during that earlier period. In addition to changes in dietary specification strategies, declining dependence on marine-origin lipid sources in recent years has placed an increased imperative on understanding the dietary need for long-chain polyunsaturated fatty acids (lcPUFA). As aquaculture continues to grow there will be an increased use of alternative lipid resources, such as grain, algal and rendered oils, to provide dietary lipids. In addition to dietary dilution of natural EFA sources through the use of these raw materials, they will also bring new challenges, such as increased levels of n-6 and 18-C polyunsaturated fatty acids (PUFA). Introduction of these n-6 and PUFA fatty acids to the diet of aquaculture species will not only influence the nutritional demands of these animals, but will also affect their flesh quality characteristics by reducing their level of n-3 lcPUFA. This dilemma will demand an increased prioritisation on the value of lipid sources rich in n-3 lcPUFA, but is also likely to stimulate the development of alternative sources of lcPUFA.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1753-5131.2009.01006.x

Exploring the nutritional demand for essential fatty acids by aquaculture species

Fish immune system and its nutritional modulation for preventive health care

Under intensive culture conditions, fish are subject to increased stress owing to environmental (water quality and hypoxia) and health conditions (parasites and infectious diseases). All these factors have negative impacts on fish well-being and overall performance, with consequent economic losses. Though good management practices contribute to reduce stressor effects, stress susceptibility is always high under crowded conditions. Adequate nutrition is essential to avoid deficiency signs, maintain adequate animal performance and sustain normal health. Further, it is becoming evident that diets overfortified with specific nutrients [amino acids, essential fatty acids (FAs), vitamins or minerals] at levels above requirement may improve health condition and disease resistance. Diet supplements are also being evaluated for their antioxidant potential, as fish are potentially at risk of peroxidative attack because of the large quantities of highly unsaturated FAs in both fish tissues and diets. Functional constituents other than essential nutrients (such as probiotics, prebiotics and immunostimulants) are also currently being considered in fish nutrition aiming to improve fish growth and/or feed efficiency, health status, stress tolerance and resistance to diseases. Such products are becoming more and more important for reducing antibiotic utilization in aquafarms, as these have environmental impacts, may accumulate in animal tissues and increase bacterial resistance. This study reviews knowledge of the effect of diet nutrients on health, welfare and improvement of disease resistance in fish.

Nutrition and health of aquaculture fish.

Fish Oils in Aquaculture: In Retrospect Sena S. De Silva, David S. Francis, and Albert G. J. Tacon Lipids in Aquafeeds J. Gordon Bell and Wolfgang Koppe The World's Oils and Fats Frank D. Gunstone Palm Oil and Saturated Fatty Acid-Rich Vegetable Oils Wing-Keong Ng and Veronique Gibon Soybean Oil and Other n-6 Polyunsaturated Fatty Acid-Rich Vegetable Oils Paul B. Brown and Steven D. Hart Rapeseed (Canola) Oil and Other Monounsaturated Fatty Acid-Rich Vegetable Oils Giovanni M. Turchini and Rodney J. Mailer n-3 Polyunsaturated Fatty Acid-Rich Vegetable Oils and Blends Douglas R. Tocher, David S. Francis, and Keith Coupland Terrestrial Animal Fats Dominique P. Bureau and David L. Meeker Alternative Marine Resources Rolf-Erik Olsen, Rune Waagbo, Webjorn Melle, Einar Ringo,and Santosh P. Lall New Alternative n-3 Long-Chain Polyunsaturated Fatty Acid-Rich Oil Sources Matthew R. Miller, Peter D. Nichols, and Chris G. Carter Minor Components in Fish Oil and Alternative Oils with Potential Physiological Effect Jana Pickova, Sabine Sampels, and Marc Berntsen Fish Oil Replacement in Starter, Grow-Out, and Finishing Feeds for Farmed Aquatic Animals Brett D. Glencross and Giovanni M. Turchini The Effects of Fish Oil Replacement on Lipid Metabolism of Fish Bente E. Torstensen and Douglas R. Tocher Welfare and Health of Fish Fed Vegetable Oils as Alternative Lipid Sources to Fish Oil Daniel Montero and Marisol Izquierdo The Effects of Fish Oil Replacement on Nutritional and Organoleptic Qualities of Farmed Fish Grethe Rosenlund, Genevieve Corraze, Marisol Izquierdo, and Bente E. Torstensen Index

Fish oil replacement and alternative lipid sources in aquaculture feeds

To examine the influence of the dietary ratio of (n-3) to (n-6) polyunsaturated fatty acids (PUFAs) on the immune system of Atlantic salmon, Salmo salar L., two dietary trials were carried out in which parr were maintained on diets containing either fish oil [(n-3)/(n-6) PUFA = 5.2] or sunflower oil [(n-3)/(n-6) PUFA = 0.3] and assessed for differences in immunological parameters. There were no significant differences in blood cell counts, differential leucocyte counts or haematocrit values between dietary groups, and while no apparent differences were observed in the non-specific immune parameters measured, there was a significantly higher number of B cells responding to Aeromonas salmonicida, in the kidney and spleen of vaccinated fish maintained on high (n-3)/(n-6) PUFAs diets. There was also a significant difference (P≤ 0.01) between the dietary groups in trial 1 and trial 2 when non-vaccinated fish were challenged with Aeromonas salmonicida and Vibrio anguillarum, respectively, with the (n-6) group succumbing to the bacterium before the (n-3) group. The results suggest that Atlantic salmon fed diets with a low ratio of (n-3)/(n-6) PUFA may be less resistant to infection than those fed diets containing lipid with a high (n-3)/(n-6) PUFA ratio.

M.F. Tatner

Papers

Effects of dietary (n-3) and (n-6) polyunsaturated fatty acid ratio on the immune response of Atlantic salmon, Salmo salar L.