Partial replacement of fishmeal and fish oil by algal meals in diets of red drum Sciaenops ocellatus

In a 6-week feeding trial with juvenile red drum, Sciaenops ocellatus (mean initial weight of 2.3 +/- 0.1 g), a control diet was formulated to contain 40% crude protein, with equal contributions from fishmeal (FM) and soy protein concentrate (SPC) as the protein sources, and 10% crude fat, using fish oil (FO) as the main lipid source. Then, in five experimental diets, 10, 20, 30, 40 and 50% of FM, SPC, and FO in the control diet was replaced by combinations of meals from the freshwater autotroph Arthrospira sp. and the heterotroph thraustochytrid Schizochytrium limacinum. In a sixth diet, FO was completely replaced by S. limacinum meal. Growth, feed utilization, and the majority of the body condition indices evaluated in fish fed the microalgae meals were not significantly (P > 0.05) different from that of fish receiving the control diet. While slight changes were observed in the proximate composition of whole fish after receiving the test diets, increased proportions of 22:6n - 3 and 16:0, alongside decreases of 18:1n - 9, 18:1n - 5, and 18:2n - 6 were observed in whole body, muscle tissue and liver of fish, which mirrored the diet composition. Overall, results of the present study indicate that up to 50% of dietary FM, SPC, and FO can be replaced by Arthrospira sp. and S. limacinum meals without compromising fish performance. In addition, S. limacinum meal can be used as the chief source of lipid for this species. In terms of absolute inclusion levels of the microalgae meals, the diet with the highest level of FM-FO replacement (50%) contained 28.87% Arthrospira sp. and 6.72% S. limacinum meals (35.59% altogether). While this is already a considerable level of incorporation of algal meals in diets for a carnivorous fish, there seems to be scope for further FM-FO replacement in the diet of red drum.