WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2014 37 of the issue is growing among nutritionists, two inevitable developments might aggravate the risk of inflammatory disorders in cultured fish. Raw material availability. Diets rich in fish oil have strong anti-inflammatory properties but these benefits are less likely to be found in future aquafeeds. Enabling further double-digit growth rates of aquaculture will require ongoing reductions in the incorporation of fish oil in aquafeed formulation, as has already been observed in recent years (Fig. 2). Fish oil is mainly produced from small pelagic fish species and the global capture of these species has increased 7-fold since the 1950s but has stabilized at approximately 20-25 million t within the last two decades. Future increases in aquaculture production will evidently require efficient replacement of fish oil, if high-energy diets with current lipid contents of up to 40 percent in salmonid feeds are to be produced (Pettersson 2010). Fish oil is an excellent source of polyunsaturated fatty acids (PUFA), key substances that reduce intestinal inflammation. Although substitution by vegetable oils might improve the bottleneck of oil availability and generate benefits in aquaculture sustainability, the fatty acid concentrations of both oil sources are different. Many oils of plant origin are rich in linoleic acid (18:2n-6), oleic acid (18:1n-9) and partially α-linoleic acid (18:3n-3). However, they lack the highly unsaturated fatty acids (HUFA), fatty acids with ≥ 20 C20 and ≥ 3 double bonds (Sargent et al. 1989). Breeding and improved genetics. Enhanced growth performance is linked to increased metabolic formation of reactive oxygen compounds that can induce oxidative stress and lead to the development of chronic inflammation. Compared to many land-based livestock, a number of the most important cultured aquatic species have been domesticated during the last decades or only relatively recently included in breeding programs. Atlantic salmon breeders can already look back on some decades of successful work, with a gradual inclusion of functional traits, such as disease resistance (furunculosis, infectious salmon anaemia), fat content and fat distribution or body shape. Further genetic improvement with higher growth performance or feed efficiency can be expected for most cultured aquatic species (Gjøen and Bentsen 1997, Ponzoni et al. 2008). The electron transport chain, a central part of the process of making dietary energy available to the organism, is an important source of free radicals, such as reactive oxygen. Increases in energy metabolism, resulting in faster growth rates, are closely related to an elevated production of these reactive oxidants, which will increase the development of inflammatory effects. Potential of Anti-inflammatory Additives One promising approach to controlling the frequent chronic form of inflammation—and thus to improving nutrition, performance and health of fish—is the inclusion of special plant-derived ingredients with strong anti-inflammatory properties in diets of animals. Although future research must close some gaps in knowledge, such as absorption and bioavailability of the substances, a growing number of in vitro and in vivo trials have shown the benefits of such additives. Flavonoids, secondary plant metabolites found in nearly all higher plants, are an interesting group of naturally active substances. The compounds consist of two aromatic and one heterocyclic ring and can be divided in different sub-classes wherein flavan derivatives exhibit strong anti-inflammatory properties (Fig. 3). The sub-classes differ in the functional group of the ring system. The main effect of flavonoids is an inhibition of the activation of the protein complex NF-ĸB, which is considered a central regulator of inflammation at the cellular level. Other mechanisms might contribute to the amelioration of inflammatory actions, inasmuch as flavonoids also are related to a reduced metabolic production of reactive oxidants. Flavonoids have good processing properties because they are relatively heat stable during practical aquafeed manufacturing (Wuerzbach et al. 2014). The anti-inflammatory effects of flavonoids have been demonstrated repeatedly in studies investigating a variety of samples, such as immune, blood or intestinal cells (Gonzales et al. 2011). So far, most information is available on the benefits of flavonoids derived from green tea and red grapes (Fig. 4) and most flavonoid TOP, FIGURE 3. Sub-groups of polyphenols. MIDDLE, FIGURE 4. Red grapes, a good source of flavonoids (Photos: Susanne Kirwan). BOTTOM, FIGURE 5. Sturgeon used for commercial caviar production (Photos: Susanne Kirwan). (CONTINUED ON PAGE 38)
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