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The effect of dietary levels of citral in juvenile common snook (Centropomus undecimalis) was evaluated by determining biomarkers of oxidative stress and innate immune system, as well as liver parameters. The fish (2.75 ± 0.03 g) were fed diets containing different concentrations of citral (0, 0.44, 0.88, and 1.76 g/kg feed) for 45 days. Results indicate that citral does not cause significant hepatic alterations; nevertheless, this monoterpenoid was able to induce lipoperoxidation in the gills in all concentrations tested. In this context, the altered activity of enzymes such as superoxide dismutase (SOD) in the liver and gills (1.76 g per kg feed), catalase (CAT) in the liver (0.44; 0.88 g per kg feed) and gills (all groups), and glutathione-S-transferase (GST) in the liver (0.44 g per kg feed) and gills (1.76 g per kg feed) occurred in response to the oxidative damage evidenced. Although dietary levels of citral did not influence the lysozyme activity, we verified that the haemolytic activity of the complement system was excessive in the lowest concentration tested and that the myeloperoxidase activity (MPO) decreased in all groups. Thus, although this monoterpenoid is a diet additive which has already been approved for consumption in other animal species, dietary supplementation with citral for common snook has not improved oxidative parameters and innate immunity of this species.

The adaptation of common snook Centropomus undecimalis to low salinities and freshwater has being widely studied, but the definition of the best water hardness levels may facilitate the adaptation and growth of this species in freshwater. Consequently, the aim of this study was to evaluate survival, growth and metabolic parameters (blood glucose and osmolality and lactate in plasma and tissues) of common snooks raised in freshwater with different water hardness levels. In an acute experiment, juveniles were transferred from seawater 35 ppt to seawater 35 ppt (control) and freshwater at 20, 100, 250, 500 or 1000 mg CaCO 3 L − 1 . Mortality after 96 h was 100, 79.17, 58.33, 4,17 and 50%, respectively. In another experiment juveniles (initial weight 10.9 ± 1.5 g) were maintained for 60 days in water hardness of 100, 500 or 1000 mg CaCO 3 L − 1 or seawater (control). No mortality was observed in this experiment. The best weight gain, specific growth rate and feed intake in freshwater (values not significantly different from seawater) was observed in common snooks kept at 100 mg CaCO 3 L − 1 . Blood and hepatic glucose did not differ significantly between treatments. Muscle glucose levels were significantly lower in fish kept at 100 and 1000 mg CaCO 3 L − 1 than the other treatments. However, lactate levels were higher in the muscle of common snooks kept at 100 to 500 CaCO 3 mg L − 1 and lactate in plasma was higher in those maintained at 1000 mg CaCO 3 L − 1 . Plasma osmolality was not affected significantly by treatments. In conclusion, 100 mg CaCO 3 L − 1 is the best water hardness to raise common snooks in freshwater.

Essential oils of plants whose main compound is citral showed beneficial effects when added to fish feed. The objective of the present study was to evaluate the dietary effect of the addition of citral on zootechnical parameters and digestive enzyme activities of Centropomus undecimalis. Juveniles were fed for 45 days with diets containing different amounts of citral (0.0 - control, 0.5, 1.0, and 2.0 mL per kg of diet). The water quality parameters were kept stable during the experiment and no mortality was observed. At the end of the experimental period, the treatment 0.5 mL citral per kg of diet had the lowest weight gain and specific growth rate, and the highest feed conversion, while the same parameters did not differ between the other treatments. Pepsin activity was higher in the stomach of fish fed with 0.5 mL citral per kg of diet and amylase activity was higher in the intestine of fish fed with 0.5 and 2.0 mL citral per kg of diet compared to the control group. Intestinal lipase activity was higher in all groups that were fed with citral compared to the control group. Chymotrypsin and trypsin activities showed no difference between groups. Consequently, dietary addition of citral at any of the levels tested is not recommended for common snook. RESUMO: Óleos essenciais de plantas, cujo composto majoritário é o citral, mostraram efeitos benéficos quando adicionados à ração para peixes. O objetivo do presente estudo foi avaliar o efeito da adição de citral sobre os parâmetros zootécnicos e atividades das enzimas digestivas de Centropomus undecimalis. Os juvenis foram alimentados por 45 dias com dietas contendo diferentes quantidades de citral (0,0 - controle, 0,5, 1,0 e 2,0 mL por kg de ração). Os parâmetros de qualidade da água foram mantidos estáveis durante o experimento e nenhuma mortalidade foi observada. Ao final do período experimental, o tratamento 0,5 mL citral por kg de ração teve o menor ganho de peso e taxa de crescimento específico e a maior conversão alimentar, enquanto os mesmos parâmetros não diferiram entre os demais tratamentos. A atividade da pepsina foi maior no estômago de peixes alimentados com 0,5 mL de citral por kg de ração e a atividade de amilase foi maior no intestino de peixes alimentados com 0,5 e 2,0 mL citral por kg de ração comparado ao grupo controle. A atividade da lipase intestinal foi maior em todos os grupos que foram alimentados com citral, em comparação ao grupo controle. As atividades de quimotripsina e tripsina não mostraram diferença entre os grupos. Consequentemente, a adição de citral na ração em qualquer um dos níveis testados não é recomendada para robalo flecha.