Effects of graded levels of a blend of Tisochrysis lutea and Tetraselmis suecica dried biomass on growth and muscle tissue composition of European sea bass (Dicentrarchus labrax) fed diets low in fish meal and oil

The aim of this study was to evaluate nutrient digestibility, growth response, carcass and fillet yields, muscle tissue composition and skin colour appearance of European sea bass (D. labrax) fed graded levels of a mixture of Tisochrysis lutea and Tetraselmis suecica freeze-dried biomass in partial substitution for fish meal and oil in diets containing substantial levels of vegetable oils and protein-rich derivatives. Five complete diets were formulated to be grossly isoproteic and isolipidic and prepared by including a blend of the two freeze-dried microalgae biomass in a 2:1 weight ratio, to replace approximately 15, 30 and 45% fish meal protein and 12, 24 and 36% fish lipid of a positive control diet. A negative control complete feed high in soybean meal was also prepared. Each diet was offered in triplicate, during 105 days until visual satiety of fish (204 ± 12.7 g) kept in a semiclosed recirculating marine water system ensuring optimal water quality to E. sea bass. The results of the study have shown that replacing about 45% crude protein and 36% lipid from fish meal and lipid by a mixture of Tisochrysis lutea and Tetraselmis suecica dried biomass, did not adversely affect growth performance and feed conversion efficiency of European sea bass. A slight decline in dry matter, protein and energy digestibility occurred in response to graded levels of dietary microalgae biomass, which was compensated by increased feed intake. Moreover, the diet including the dried microalgae resulted in a higher nutritive value than that of the negative high-soybean meal control feed. No major changes were observed in biometry traits and slaughter yield while the nutritional properties of the edible muscle tissue were little affected in terms of n-3 PUFA. The presence of the dried microalgae in the diet resulted in a greenish pigmentation of the skin, with a slight tendency towards redness and diminished lightness and hue. Overall the results obtained here reveal a potential improvement in sustainability in terms of reduced reliance on halieutic feed resources in case of the microalgae-containing diets. Hence the sustainable use of the dried mixture of the 2 marine microalgae biomass as feed ingredients in diets for the E. sea bass seems to be mainly limited by the low availability and unaffordable market price which are both expected to improve over the next 10 years.