As an abundant source of antioxidants and diet flavor enhancers, the plant essential oils can have positive effects on fish growth, and resistance against environmental stressors. In this study, garden thyme (Thymus vulgaris) essential oil (TEO) was used in the diet of rainbow trout, Oncorhynchus mykiss, to evaluate its protective effect against malathion pesticide exposure. Tested fish (19.99±0.01 g) were divided into six groups (three replicates), namely: T1: control diet; T2: control diet + 0.025 mg L−1 malathion; T3: control diet + 0.075 mg L−1 malathion; T4: control diet + 1% TEO; T5: control diet + 0.025 mg L−1 malathion + 1% TEO and T6: control diet + 0.075 mg L−1 malathion + 1% TEO. After 21 days, T4 fish had the highest final body weight (FW), weight gain (WG), specific growth rate (SGR), and the lowest feed conversion ratio (FCR) among experimental treatments (P<0.05). The blood parameters including the red blood cells (RBC), white blood cell count (WBC), hematocrit (Hct), and hemoglobin (Hb) values were the highest in T4 treatment, displaying a significant difference with T1 treatment (P<0.05). Fish in the T4 groups had the highest total protein (TP) and albumin (ALB), while fish of T3 showed the lowest levels of these parameters (P<0.05) and also had the highest level of triglycerides (TRG), cholesterol (CHOL), lactate dehydrogenase (LDH), and urea (Ur). Alanine aminotransferase (ALT), aspartate aminotransferase (AST), and alkaline phosphatase (ALP) enzymes recorded the lowest levels in T4 treatment, which showed a significant difference with T1 group. The catalase (CAT) and superoxide dismutase (SOD) showed the highest activities in T4 treatment, while the lowest SOD and the highest malondialdehyde (MDA) levels occurred in T3 group (P<0.05). Total immunoglobulin (total Ig) level, alternative complement (ACH50) and lysozyme in the serum and skin mucus of T4 treatment of rainbow trout showed the highest activities with a significant difference from groups (P<0.05). From the results of the present study, it can be concluded that 1% of T. vulgaris as a supplement to the diet of rainbow trout can stimulate and improve the immune system of the fish. TEO can have a protective effect against unfavorable effects of malathion and improves the growth of the fish.
