Behavioural and gill histopathological effects of acute exposure to sodium chloride in moneda (Metynnis orinocensis)

To evaluate the toxicity of sodium chloride (NaCl), juveniles and adult Metynnis orinocensis were exposed for 96 h to 0, 5, 10, 15, 20 or 40 g L−1 of salt. Food intake, behaviour, opercular frequency (OF), mortality, body weight and gill microscopic alterations were evaluated. Behavioural changes were observed in fish exposed to concentrations higher than 10 g L−1. Juveniles and adults showed a progressive decrease in the OF and body weight. Food intake decreased in concentrations below 15 g L−1. Juveniles and adults exposed to 15, 20 or 40 g L−1 had 100% mortality. Lamellar congestion, hyperplasia and fusion were the common microscopic alterations at higher concentrations. The gill congestion severity increased with salt concentration. The LC50 for juveniles and adults were 10.5 g L−1 and 10.8 g L−1, respectively. These results suggest that salt concentrations lower than 5 g L−1 are safe for preventive and therapeutic practices in Metynnis orinocensis; whereas prolonged exposure higher than 10 g L−1 is deleterious in this species.