World Aquaculture Magazine -December 2021

54 DECEMBER 2021 • WORLD AQUACULTURE • WWW.WA S .ORG fatty acid profile have been neglected because of a lack of knowledge on the importance of fatty acids in tilapia performance, especially at cold suboptimal temperatures. As Nile tilapia is an omnivorous fish and a tropical species, marine fishmeal or marine fish oil have been totally excluded from diet formulations in recent years because of associated increased costs, being substituted by ingredients of terrestrial origin such as plant meals and oils and poultry by- product meals, among others. Such feedstuffs are poor in long-chain polyunsaturated fatty acids of the n-3 series (also known as omega-3 fatty acids) that are essential for membrane function in suboptimal temperatures. Therefore, in addition to a loss of quality in the fillet regarding fatty acid profile, it is also expected to result in a loss of tilapia production in subtropical regions. Like other vertebrates, fish can metabolize dietary nutrients to produce body tissues and to maintain homeostasis and normal metabolic function. Although fish can obtain energy from these nutrients, there are other essential nutrients that must be obtained directly from the diet to improve development and survival of fish. In parallel to proteins that consist of amino acids, lipids in part consist of fatty acids. There are five fatty acids considered essential for fish: linoleic acid (LOA, 18:2n-6), alpha-linolenic acid (α-LNA, 18:3n- 3), arachidonic acid (ARA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) (Glencross 2009). Saturated fatty acids (SFA) have no double bonds in their carbon chain, monounsaturated fatty acids (MUFA) have only one double bond, while polyunsaturated fatty acids (PUFA) have two or more double bonds. The five essential fatty acids are PUFA of n-3 or n-6 series (ω-3 or ω-6), depending on the position of the first double bound in their chain. For example, LOA is a fatty acid of the n-6 series with 18 carbons and two double bonds described as 18:2n-6 and DHA is a fatty acid of the n-3 series with 22 carbons and six double bonds described as 22:6n-3. Although SFA andMUFA can be produced by fish metabolism, PUFA cannot and consequently must be supplied directly from the diet. However, PUFA of 18 carbons obtained from the diet can be partly elongated to PUFA of greater chain length and higher biological value to fish. Another important concept about fatty acids is that they comprise the phospholipids responsible for the structure of cell membranes of all living organisms (Fig. 5). PUFAs are important to maintain cell membrane permeability because of their configuration that organizes the lipid bilayer of the membranes, improving their fluidity, permeability and function, especially at lower temperatures because of the lower melting point of fatty acids with higher unsaturation and chain length (Fig. 6). Dietary Supplementation of n-3 Polyunsaturated Fatty Acids for Nile Tilapia Improves Growth Performance at Cold, Suboptimal Temperature Nile tilapia reared at optimal temperature have a dietary requirement for LOA (Takeuchi et al. 1983) and/or α-LNA (Chen et al. 2013) that could be supplemented in commercial feeds by plant oils (i.e., sunflower, linseed, soybean and canola oils). When Nile tilapia is reared at optimal temperature, the dietary total PUFA seems to be required at a level near of 1.30 percent dry diet, regardless of n-3/n-6 PUFA ratio (Mufatto et al. 2019). Conversely, at a cold suboptimal temperature, Nile tilapia requires a higher dietary n-3/n-6 PUFA ratio, especially with n-3 PUFA of longer chain length. Growth and feed efficiency are improved when Nile tilapia are fed diets containing fish oil that is rich in n-3 long-chain PUFA such as EPA and DHA at 22 C, when compared to tilapia fed diets with plant oil sources (Corrêa et al. 2017, 2018). Additionally, Nile tilapia reared at cold suboptimal temperature had 20 to 27 percent greater daily weight gain and 7 to 10 percent better feed efficiency when fed a diet containing fish oil than tilapia fed diets containing mixes of plant oils. In studies that evaluated α-LNA (from linseed oil) dietary requirement of Nile tilapia, it was observed higher requirement in fish reared at a cold suboptimal temperature of 22 C (0.68 to 0.70 g of α-LNA / 100 g diet dry weight, Nobrega et al. 2017), than in fish reared at optimal temperature (0.45 to 0.64 g of α-LNA / 100 g diet dry weight; Chen et al. 2013). In both studies, total dietary PUFA content was similar at 1.23 and 1.25 g / 100 g dry weight. In light of these recent studies, it is important to find alternative sources of n-3 long-chain PUFA to supplement winter or pre- winter diets for Nile tilapia reared in subtropical regions. Algae and heterotrophic microorganismmeals seems to be good substitutes to the meal and oils from overexploited marine fish as n-3 long chain FIGURE 4. Adequate fatty acid composition in commercial diets have the potential to improve growth, nutrient digestibility, feed efficiency and fish health. FIGURE 5. Cell membranes are composed of a bilayer of phospholipids that have a hydrophilic head (phosphate) and a hydrophobic tail (fatty acids).