20 SEPTEMBER 2016 • WORLD AQUACULTURE • WWW.WAS.ORG the cost-effectiveness and sustainability of commercial shrimp feed. However, soybean meal has a different nutrient and amino acid profile than fishmeal. Reduced palatability and antinutritional factors causing low digestibility can lead to impaired culture performance of shrimp. Moreover, fishmeal is of significant dietary value to shrimp as a source of minerals and omega-3 longchain fatty acids that are absent in soybean meal. In contrast to marine lecithins that contain omega-3 fatty acids in phospholipid form, soy lecithins are particularly rich in the omega-6 fatty acid, linoleic acid. Krill meal is rich in chitin from the exoskeleton and soluble compounds such as trimethylamine oxide, free amino acids and nucleotides that may act as feeding stimulants (Table 1). Feeding stimulation by partial replacement of fishmeal by krill meal in diets leads to increased growth of shrimp, which also occurs in situations of feeding depression (e.g. high plant-based diets) or stress conditions (e.g. change in salinity or temperature). Krill meal is also used in functional fish feeds to increase growth and fillet quality. Salmon fed a diet that included krill mill had reduced mortality and heart pathology during viral infection. Further study is needed to determine if krill meal has a similar effect on the immune system of shrimp. Krill meal also contains around 25 percent lipids that include a high amount of phospholipids (around 40 percent), omega-3 fatty acids (EPA and DHA) and astaxanthin esters. The antioxidant astaxanthin provides pigmentation and is known to have anti-inflammatory properties by scavenging free radicals, thereby counteracting oxidative stress and damage to proteins, lipids and DNA (Sies 1993). In fish oils, the omega-3 fatty acids are bound to triglycerides; in krill meal, these fatty acids are bound to phospholipids (Fig. 6). The structural difference in presenting omega-3 fatty acids might be of relevance to shrimp health because omega-3 fatty acids that bind to phospholipids, the building blocks of all cells, are more effectively incorporated into tissues and cells than omega-3 bound to triglycerides (Graf et al. 2010, Liu et al. 2014, Rossmeisl et al. 2012, Wijendran et al. 2002). The main phospholipid in krill meal is phosphatidylcholine (Phleger et al. 2002, Tou et al. 2007), which not only delivers omega-3 fatty acids, but also choline. Choline is an essential vitamin-like nutrient that is crucial for cellular functions such as neurotransmission and osmoregulation (Food and Nutrition Board 1998). Choline must be added to shrimp feed unless sufficient phosphatidylcholine is present (Gong et al. 2000). Phospholipids might also help overcome the limited ability of shrimp to make endogenous phospholipids (Kanazawa et al. 1985). Shrimp depend on dietary phospholipids, especially in larval and juvenile stages, for building cell membranes, lipid digestion and transport in plasma lipoproteins, and cell signaling. Phospholipids are particularly abundant in the hepatopancreas. Sufficient energy stored in the hepatopancreas is important for shrimp growth and improves shrimp’s ability to resist to stressful conditions (e.g. low oxygen concentration, molting, rapidly changing water temperature or salinity etc.). Dietary phospholipids ensure that sufficient energy is stored in the hepatopancreas and fats can be mobilized for transport to tissues, particularly important to overcome stressful environmental changes (Gong et al. 2004). Cholesterol is also a vital nutrient in shrimp feed, important FIGURE 4. Intensive grow-out pond for shrimp in Indonesia using paddlewheel aerators. FIGURE 6. Omega-3 fatty acids can be in phospholipid or triglyceride forms. FIGURE 5. The amount of wild harvested forage fish for fishmeal has remained constant since the 1990s, while aquaculture production has been increasing steadily.
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