Compensatory fatty acid metabolism and hepatic gene expression in African catfish (Clarias gariepinus) fed marine-ingredient-free circular diets low in EPA and DHA

Abstract: Raw materials rich in EPA (C20:5n-3) and DHA (C22:6n-3) such as marine fish oils and meals are becoming increasingly scarce and valuable throughout the aquafeed industry, while human dietary intake of EPA and DHA is deficient in many parts of the world. Therefore, EPA and DHA sources have to be used judiciously and strategically in diets for especially freshwater fish, which, compared to their marine counterparts, tend to have a superior ability to biosynthesize these functionally important fatty acids (FAs) from their C18 precursors. Hence, this study tested the effect of a minimal dietary EPA+DHA level on growth performance, filet and liver FA profile development, gene expression dynamics (elovl2, elovl5, fads2) and nutrient excretion of African catfish (Clarias gariepinus) by replacing salmon oil with rapeseed oil in a marine-ingredient-free basal diet formulated with low EPA+DHA circular animal protein sources. Even though growth performance attained with a commercial African catfish diet utilizing the same salmon oil was not achieved with the two experimental diets, low EPA+DHA levels in the rapeseed oil experimental diet did not negatively affect growth. Expression of hepatic elovl2, elovl5 and fads2 was significantly elevated in the rapeseed oil fed fish while particularly EPA and DHA in the filet and EPA in the liver were consistently and significantly lower compared to the salmon oil-based diets. Concomitant examination of gene expression and FA profile development indicates the notable capacity of African catfish to de novo synthesize EPA and DHA from ALA (C18:3n-3), which suggests the potential of low EPA+DHA diets for this species. From a human nutritional point of view, however, this biosynthetic capacity was insufficient to compensate for the effect of the low EPA+DHA rapeseed oil diet on filet FA profile quality, with EPA+DHA content and n-6:n-3 FA ratio being significantly negatively affected. Irrespective of oil source, the phosphorus-rich experimental diets resulted in a significantly improved ratio of excreted dissolved nitrogen to phosphorus compared to the industrial control from a plant nutrition perspective, confirming prior results on the ability to improve diets for use in aquaponics.