World Aquaculture Magazine -December 2021

WWW.WA S .ORG • WORLD AQUACULTURE • DECEMBER 2021 55 PUFA sources in the diets for farmed fish. Aurantiochytrium sp. is a heterotrophic thraustochytrid microorganismwith rapid growth and, when dried, it is relatively easily processed into Aurantiochytrium meal (AM), a source of commercially available DHA. Nile tilapia maintained at 22 C responded positively to increasing dietary inclusions of AM, improving growth performance. Dietary supplementation of AM in the range of 0.45 to 1.42 g / 100 g dry diet weight was sufficient to provide the greatest growth, feed efficiency, body lipid composition, and n-3/n-6 PUFA ratio in fish muscle (Nobrega et al. 2019). Dietary supplementation of 1 g AM / 100 g dry diet weight promoted 5 percent greater growth than the inclusion of 2 g / 100 g cod liver oil and also promoted 16 percent greater growth in comparison to fish fed a diet without DHA supplementation (Nobrega et al. 2019). Many studies have reported that dietary n-3 PUFA can increase productivity at suboptimal temperature (Corrêa et al. 2018, Abdel- Ghany et al. 2019, Nobrega et al. 2019). However, it is important to adequately formulate commercial feeds to avoid dietary excess of n-3 PUFA because of probable negative effects on growth performance and immune response of fish. Nile tilapia fed excess dietary α-LNA had reduced growth and feed utilization regardless of water tempera- ture (Chen et al. 2013, Nobrega et al. 2017). In addition, immunosup- pressive effects of excess long-chain PUFA have been reported in freshwater fish (Fracalossi and Lovell 1995, Chen et al. 2016). Dietary Fatty Acids Influence Diet Digestibility A diet rich in saturated fatty acids (SFA) impairs growth of Nile tilapia reared at suboptimal temperatures (Takeuchi et al. 1983, Ng et al. 2004, Turchini et al. 2009, Corrêa et al. 2017, Abdel-Ghany et al. 2019). In lab studies, we compared growth of juvenile Nile tilapia reared at 28 C and 22 Cwhen fed different lipid sources (Fig. 7) (Corrêa et al. 2017). Fish fed a diet with coconut oil, rich in SFA, had a 90 percent reduction in weight gain with the temperature decrease, whereas fish fed a diet with fish oil, rich in long-chain n-3 PUFA, had only 67 percent reduction in weight gain. Other plant oils or mixes of plant oils, including sunflower and linseed oils, led to a 67-74 percent growth reduction with the temperature decrease (Corrêa et al. 2018). Complementary data, not yet published, suggest that Nile tilapia performance impairment was related to poor digestibility of the lipid if coconut oil was the sole lipid source in the diet, especially for fish reared at 22 C. In previous studies, within the saturated fatty acids, the digestibility of palmitic acid (16:0) decreased from 71 percent at 28 C (Fernandes et al. 2019) to 52 percent at 22 C (Nobrega et al. 2019). Diets rich in SFAmay lose less fat to water because of the high melting point of such fatty acids, but digestibility will also be impacted. Indeed, saturated fatty acids contribute to the decrease of oil fluidity, increasing viscosity, thus negatively affecting digestibility and metabolism in fish (Nobrega et al. 2019). AMprotein and lipid digestibility in Nile tilapia at 22 C (Nobrega et al. 2019) decreased by around 20 percent when compared to fish fed at 28 C (Fernandes et al. 2019). In contrast, all PUFA resulted in a high digestibility coefficient, at both optimal and suboptimal temperatures. At 22 C, all PUFA in AM showed high digestibility by Nile tilapia, ranging from 96 to 100 percent (Nobrega et al. 2019). Similar results were also reported in Atlantic salmon maintained at 8.8 C, a temperature less than optimal for this species (Kousoulaki et al. 2016). Final Considerations There has been a shortage of n-3 PUFA-rich ingredients in commercial feeds for tilapia owing to cost constraints. However, the practice of limiting these ingredients should be reviewed, especially for winter diets or pre-winter diets in subtropical regions, because dietary supplementation of n-3 PUFA at adequate amounts improves growth performance, feed efficiency and prevents mortality of Nile tilapia at low temperatures. In addition, the increase of n-3 PUFA in the diets of Nile tilapia will improve fillet quality, especially EPA and DHA content, adding nutritional and commercial value to the product and meeting consumer demand. Outdoor studies at a large scale are planned by our research team to validate the costs and benefits of supplementation of a source of n-3 PUFA in winter diets for Nile tilapia. Likewise, lab studies are underway to investigate if other membrane components such as phospholipids and/or cholesterol would help tilapia face the winter in subtropical regions. FIGURE 6. Monounsaturated and polyunsaturated fatty acids improve cell membrane function at suboptimal growing temperatures. FIGURE 7. Oil sources vary greatly in their fatty acid composition. ( C O N T I N U E D O N P A G E 5 6 )