Replacement of fish oil by soybean oil and microalgal meals in diets for Totoaba macdonaldi (Gilbert, 1890) juveniles

The objective of this study was to evaluate the use of soybean oil (SO) and two microalgal meals derived from Schizochytrium limacinum (SL1 and SL2) to replace fish oil (FO) in diets for Totoaba macdonaldi juveniles. Nine isoproteic (54% crude protein, CP) and isolipidic (14% crude fat, CF) diets were formulated based on a 3 × 3 factorial design, using the three lipid sources and three replacement levels, 33%, 66% and 100%. A control diet containing FO as the main lipid source served as a reference. Diets were fed for 6 weeks and were randomly assigned to six replicate tanks of a recirculating aquaculture system, with 25 fish stocked per tank. Initial mean fish weight was 8.08 g; at the end of the trial, statistical differences for growth responses such as weight gain (WG; P = 0.0069) and thermal growth coefficient (TGC; P = 0.0027) were detected. WG (SL1 = 50.75 g, SL2 = 51.43 g) and TGC (0.148) of fish fed the S. limacinum meals were significantly higher than in those fed SO (WG = 41.13 g; TGC = 0.129). Conversely, the hepatosomatic index (HSI; P = 0.0009) of fish fed SO was significantly higher (1.38%) compared to those fed the microalgae meals (SL1 = 1.13%, SL2 = 1.22%). Also, proximate composition of liver was significantly influenced by the lipid source and CF was significantly higher (P = 0.0067) in fish fed SO (47.68%) than in fish fed SL1 (37.44%) or SL2 (39.86%). The fatty acid (FA) profile of muscle and liver reflected the dietary FA composition; SO prompted increased concentrations of 18:2n-6 and total n-6 FA in fish fillet; whereas, the S. limacinum meals increased 22:6n-3, total n-3 FA and the n-3/n-6 ratios, which is highly desirable in food destined for human consumption. In addition, the concentration of pancreatic lipase 1 (70.2 kDa) in anterior intestine and pyloric caeca of fish significantly decreased (P = 0.0019) from 2.93 to 1.85 mg g as the level of FO replaced increased. These findings indicate that up to 100% of FO can be replaced by S. limacinum meals improving the growth of T. macdonaldi and preserving the nutritional value and the health benefits of the fillet for human consumption; whereas, SO decreased growth and the concentration of 22:6n-3, 20:5n-3, total n-3, and n-3/n-6 ratios, while increasing the HSI and the risk of hepatic dysfunction and steatosis. −1