Effect of carvacrol on the haemato‐immunological parameters, growth and resistance of Colossoma macropomum (Characiformes: Serrasalmidae) infected by Aeromonas hydrophila

The aim of the present investigation was to evaluate the effect of carvacrol, administered with feed, on growth, haematology and survival tambaqui, Colossoma macropomum , infected by Aeromonas hydrophila exposed to hypoxia. Carvacrol was incorporated into fish feed at the following concentrations: 0 (control), 10, 20 and 40 g.kg −1 for 60 days. Fish fed 10 g.kg −1 of carvacrol‐enriched diet had an increase in final weight (72.68 ± 1.63) and specific growth rate (1.89 ± 0.03) and a decrease in apparent feed conversion compared (1.65 ± 0.09) with the control group. In addition, the condition factor (2.03 ± 0.04) and weight gain (49.31 ± 1.54) were also improved in animals fed 10 g.kg −1 with carvacrol. RBC count, MCV, mean MCHC, LG‐PAS + , thrombocytes and lymphocytes presented no differences between treatments; however, Ht (25.6 ± 1.20) and mean MCV (224.05 ± 9.64) of the animals fed the highest carvacrol level were lower than the control animals, suggesting microcytic anaemia. Increased monocytes (6.54 ± 1.07), neutrophils (9.14 ± 0.57) and phagocytic activity (32.77%) were recorded in animals fed 10 g.kg −1 with carvacrol, in relation to the control. Glucose, cholesterol and albumin presented no changes; however, lower triglyceride values were observed in animals fed 40 g.kg −1 carvacrol‐enriched diet. Animals fed 10 g.kg −1 of carvacrol had higher plasma protein (3.31 ± 0.05) and globulin (1.97 ± 0.06) concentrations and a higher survival rate when submitted to bacterial infection. Thus, carvacrol‐enriched diet improves C . macropomum growth, haemato‐immune system, metabolism and resistance to A . hydrophila infection.