Growth performance and quality traits of European sea bass (D. labrax) fed diets including increasing levels of freeze-dried Isochrysis sp. (T-ISO) biomass as a source of protein and n-3 long chain PUFA in partial substitution of fish derivatives

The aim of this studywas to evaluate nutrient digestibility, growth performance, biometry, dressing out parameters, fillet muscle proximate and fatty acid composition of European sea bass (Dicentrarchus labrax L.) of nearly marketable size, fed increasing levels of a freeze-dried biomass of Isochrysis sp. (clone T-ISO) as a partial substitute for protein and lipid from fish derivatives, in diets where the level of n-3 long chain (LC) PUFA were reduced through a partial replacement of fish oil for crude palm oil. Since diets including the test ingredients could possibly result in changes of certain quality attributes, fish were also subjected to overall sensory evaluation by a trained panel of assessors according to a triangle test. Three dietswere formulated to be grossly iso-nitrogenous (N, 7.5% dry matter). A preparation inwhich fish meal/ trimmings and fish oil were the major protein and lipid sources was used as a positive control diet. Two test complete diets were obtained from the control preparation so that approximately 10 and 20% crude protein from a high quality fish meal and 18 and 36% lipids from fish oil were supplied by the dried microalgae biomass. To stress the role of Isochrysis sp. as a source of n-3(LC)PUFA, the diets including increasing levels of the microalgae biomass were made grossly iso-caloric–lipidic by replacing a part of fish oil with palm oil which is known to be virtually free from n-3 PUFA. Each diet was fed to apparent satiety to quadruplicate groups of sea bass (142 g) over 121 days. The results have shown that replacing up to 20% crude protein from fish meal with a dried Isochrysis biomass and up to 36% fish lipid for those supplied by the microalgae in a diet with reduced level of fish oil, did not adversely affect feed intake or growth performance relative to controls even if the highest substitution rate resulted in a decline (P b 0.05) in lipid and energy apparent digestibility coefficients and in a slightly reduced n-3(LC)PUFA content of the edible muscle tissue (P b 0.05). No major diet-dependent changes were observed in biometry traits and slaughter yield. At the actual dietary inclusion levels, the presence of dried microalgae led to increased greenish pigmentation of the skin, but did not permit discrimination of the cooked fillets by sensory analysis using the triangle test.