The use of microalgae as dietary ingredients for marine carnivorous fish

Recent improvements in technological production systems allow massive microalgae biomass under controlled conditions to be produced. This makes such organisms a potential alternative to conventional sources of omega-3 polyunsaturated fatty acids also for feeding marine finfish species of interest for aquaculture. Their major constrain still represented by their high production cost - although expected to become more affordable in the near future - is counterbalanced by the high environmental sustainability, being microalgae at the origin of the food chain, and by the high nutritional value. Several species are actually available at an industrial scale and the potential of two marine microalgae as ingredients in aquafeeds for the European sea bass (Dicentrarchus labrax L.) is presented. Dried Tetraselmis sueccica and T-Isochrysis aff. galbana were included to replace 10% (Tetra10 & T-ISO10) and 20% (Tetra20 & T-ISO20) protein supplied by fish meal which was the major protein source in the control diet. All diets resulted isoproteic (50% DM) and isolipidic (18% DM). The formulated feeds were fed to juveniles sea bass for a minimum of 9 weeks under controlled environmental conditions in two separated trials according to completely randomized designs with three replicates per dietary treatment. Feeding the test diets resulted in similar fish growth and feed utilization as well as in the hedonistic properties of the fish fillet as evaluated by a triangular test involving a panel of 10 trained judges at the end of the trial considering T-ISO dietary inclusion. Anyway feeding diets including high levels of microalgae resulted in decreasing diet digestibility measured by in vivo trial (ADC coefficients) and in enhanced green-yellowish skin pigmentation (CIE system). Moreover despite a 60% reduction of dietary fish oil in case of T-ISO20 diet, only a slight reduction in LC-omega-3 polyunsaturated fatty acids content was measured in the flesh. Data suggest the possibility to optimize the combination of these two microalgae to meet the specific nutritional requirements to make the feeding practice even of a marine carnivorous fish more sustainable.