Fish oil replacement by a DHA-rich Schizochytrium meal in microdiets for rainbow trout ( Oncorhynchus mykiss ) larvae

Microalgae are rich in nutrients and health-promoting compounds, with great potential as aquatic feed ingredients. This trial was conducted to evaluate the effects of replacing dietary fish oil with docosahexaenoic acid (DHA) rich microalgae Schizochytrium meal for rainbow trout ( Oncorhynchus mykiss ) larvae. Five diets were prepared by using Schizochytrium meal to replace 0% (R0), 25% (R25), 50% (R50), 75% (R75), or 100% (R100) fish oil DHA in the control diet (crude protein, 52%; crude lipid, 15%). Rainbow trout larvae (initial body weight 0.25 ± 0.01 g) were fed the diets in square fiberglass tanks for 56 d. Survival, final body weight, weight gain, specific growth rate, and final body length were not significantly affected by the substitution of fish oil with Schizochytrium meal (P>0.05). Trypsin, amylase, lipase, leucine-aminopeptidase (LAP) and alkaline-phosphatase (AKP) activities were not affected by dietary treatments (P>0.05). No significant differences were observed in body C16:0, mono-unsaturated fatty acids (MUFA), SFA (saturated fatty acids), DHA and n-3 fatty acids content in fish among dietary treatments (P>0.05). C14:0, C16:1n-7, eicosapentaenoic acid (EPA), C20:4n-6 content, and n-3/n-6 ratio significantly decreased, while C18:0, C20:0, C18:1n-9 content, DHA/EPA ratio significantly increased in fish as Schizochytrium meal replacement level increased from 0% to 100% (P<0.05). C18:3n-3, C18:2n-6, polyunsaturated fatty acids (PUFA) and n-6 fatty acids content in fish increased with increasing Schizochytrium meal replacement level (P<0.05). There were no statistically significant differences in intestinal fold height, enterocyte height and microvillus height among all test treatments (P>0.05). The results of this study denoted the good nutritional value of DHA rich Schizochytrium meal as a replacement of fish oil in microdiets without negatively affecting survival, growth, and activities of digestive enzyme rainbow trout larvae.