This study evaluated the use of microalgae (Aurantiochytrium sp.) meal as a substitute for fish oil in the diet of juvenile Pacific white shrimp (Litopenaeus vannamei) reared in a clear‐water system. Dietary treatments at five replacement levels (0%, 25%, 50%, 75%, 100%) were performed in triplicate. After 46 days, only a slight difference in shrimp final weight was observed among treatments (0.61 g). An increase in final weight was observed with replacement of up to 50% fish oil for microalgae meal, while the optimal percentage of replacement estimated was 44.7%. Feed conversion rate (FCR) of animals fed a diet of Aurantiochytrium sp.meal to replace up to 50% fish oil decreased, and the optimal percentage of replacement estimated was 49.3%. The fatty acids profile in shrimp muscle tissue demonstrated an increase in docosahexaenoic acid (DHA) from 10.03% to 14.28% with increased replacement of fish oil by microalgae meal in the diet. Therefore, the partial replacement of fish oil by microalgae meal resulted in improved shrimp growth and FCR, and total replacement of dietary fish oil had no negative effects on these parameters. In addition, inclusion of microalgae meal raises the level of DHA in shrimp muscle.

Proper inclusion of carbohydrates in aquafeeds can increase protein and lipid retention preventing the catabolism of these nutrients for energy purposes. This research aimed to evaluate performance, metabolic and enzymatic indicators in pacu (Piaractus mesopotamicus) juveniles fed diets with 350 or 400 g/kg starch (S) and 190 or 210 g/kg digestible protein (P) (diets S35P19, S35P21, S40P19 and S40P21 respectively). Overall, the best growth performance was achieved with diet S35P21. Dietary protein and starch levels had no effect on feed intake, feed conversion ratio and protein efficiency ratio. The lowest whole‐body protein content was recorded in fish fed the S35P19 diet, while fish fed diet S40P21 presented the highest whole‐body lipid content. Regardless of dietary protein level, S40 diets led to higher plasma glucose and triglycerides levels, and higher liver glycogen content. No differences were observed on hepatosomatic index or muscle lipids content. Increasing dietary starch level led to an increase in mesenteric fat index and in liver glucokinase activity, while higher dietary protein promoted a decrease in liver lipid content. Fish fed diet S40P21 showed higher liver glucose‐6‐phosphate dehydrogenase activity than fish fed the S35P21 and S40P19 diets. Dietary protein and starch levels had no effect on pyruvate kinase and fructose‐1,6‐bisphosphatase activity. Overall, dietary starch enhanced glycolytic and lipogenic pathways but did not depress gluconeogenic pathway. The results of this study indicate that a diet with 350 g/kg starch and 210 g/kg digestible protein was the most adequate for pacu in terms of growth performance and feed utilization.