Replacement of fishmeal with processed canola meal in diets for juvenile Nile tilapia (Oreochromis niloticus): Growth performance, mucosal innate immunity, hepatic oxidative status, liver and intestine histology

The present study was carried out to evaluate the effect of processed canola meal (PCM) as fishmeal (FM) substitute in juvenile Nile tilapia diet. Firstly, canola meal (CM) was dephytinized using 2000 IU/kg phytase and subjected to methanolic ammonia solution (MAS). Finally, five isonitrogenous and isoenergetic diets were formulated by inclusion of 0 (control), 12.5, 25, 37.5 and 50% PCM at the expense of dietary FM. Each experimental diet was fed to three randomly allotted groups of 17 juvenile Nile tilapia with an average body weight of 3.5 +/- 0.1 g for 36 days. Fish were feed three times a day until apparent satiation. Results showed that there were no statistically significant differences regarding SGR, final body weight and WG between control group and PCM2 (i.e., diet with 25% PCM inclusion level) (p > .05); however receiving diets with higher PCM content (> 25%) resulted in significantly decreased growth indices (p < .05). Meanwhile, FCR, FI and apparent digestibility coefficient of protein (ADC(protein)) did not differ among experimental groups (p > .05). Digestive enzyme activities, mucosal innate immunity, liver antioxidant enzyme activities and liver tissue malondialdehyde content (MDA) were not significantly affected by dietary inclusion of PCM (p > .05). However, relative gene expression of superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT) were significantly increased by dietary PCM content increment (p < .05). In line with growth performance, histological observations revealed that those fish received higher levels of dietary PCM content (i.e. 37.5 and 50%) showed intensive intestinal and hepatic mononuclear immune cell infiltration, lamina propria expansion and intestinal villus detachment and shortening (p < .05). In conclusion, despite containing lower glucosinolate, phytic acid, phenolic compounds and tannins well within the recommended thresholds for aquatic animals nutrition, diets containing PCM beyond 25% negatively affected the growth performance and intestine and liver tissue histoarchitecture of juvenile Nile tilapia. However, further complementary studies are needed regarding the effects observed on gut histology and expression of antioxidant enzyme genes.