Fish oil replacement with linseed oil affects growth, tissue fatty acid composition, blood chemistry, and antioxidant biomarkers in rohu, Labeo rohita

The effect of replacing fish oil (FO) with linseed oil (LnO) in rohu, Labeo rohita (initial weight 19.1 ± 0.1 g), diets was evaluated in a 90-day feeding trial. Five isonitrogenous (30% crude protein) and isolipidic (11% crude lipid) diets were prepared by substitution of FO (7% FO in the control) with LnO (0, 25, 50, 75, and 100%). Responses of the fish were tested in triplicate for each diet. Partial replacements up to 75% did not cause any significant change in growth performance and feed efficiency. However, total replacement of FO significantly reduced growth performance and feed utilization (P < 0.05). Survival, hepatosomatic index (HSI), and viscerosomatic index (VSI) as well as whole-body proximate composition were not significantly influenced by the dietary treatments. The ratio of alpha linolenic acid (ALA; 18:3n-3) increased in the muscle and liver with the increasing level of dietary LnO, while the percentage levels of eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) decreased. However, EPA and DHA were in considerable level (3.3 and 18.6%, respectively) in muscle even when fed diet with 100% LnO. Hematological parameters such as red blood cell (RBC), hemoglobin (Hb), and hematocrit (Hct) were not influenced by dietary treatments, averaging 2.2 × 10⁶ U/L, 7.6 g/L, and 27.8%, respectively. On the other hand, alanine transaminase (ALT) and aspartate transaminase (AST) activity in serum were significantly elevated with the LnO inclusion levels (P < 0.05), reaching the highest levels (33.0 and 310.7 U/L, respectively) when LnO totally replaced FO. Cholesterol (CHOL), triacylglycerol (TG), high-density lipid (HDL), and low-density lipid (LDL) levels were significantly enhanced (P < 0.05) compared to the control. Cortisol (CORT) and glucose (GLU) were not significantly changed, averaging 1.5 nmol/mL and 20.8 g/dL, respectively. In the liver and serum, superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) activities were significantly reduced, while malondialdehyde (MDA) level was significantly increased (P < 0.05) with the dietary LnO inclusion. Our results indicate that FO can be substituted with LnO up to 75% in practical diet formulations for rohu without any considerable negative effect, which will substantially decrease the dependency on dietary FO.