The effect of acute exposure to different water pH levels on urinary excretion and plasma ion levels in silver catfish Rhamdia quelen was analysed. Fish were exposed to pH 4Á0, 5Á0, 7Á5, 8Á0, and 9Á0 for 4 days and urine was collected. Other specimens were also exposed to the experimental pH for 24 h and blood was sampled. Urine flow rate, urine and plasma pH showed a significant trend to increase with the increase of water pH. Urinary Na þ excretion rate also increased and ammonia urinary excretion rate decreased with the increase of water pH. There was a significant trend to decrease volume, ammonia, Cl À and Na þ urinary excretion rate with increasing mass in fish exposed to all pH levels studied. Plasma ammonia levels showed a slight decrease in fish exposed to water pH from 4Á0 to 8Á0, but those exposed to water pH 9Á0 presented the highest ammonia levels. Most plasma ions and urinary excretion changes observed in silver catfish exposed to acidic or alkaline water were similar to those already detected in rainbow trout Oncorhynchus mykiss. In addition, the kidney and urinary bladder might participate on acid-base balance in silver catfish, since urine pH changed according to plasma pH.

No abstract

Low dissolved oxygen levels in the water (hypoxia) can be provoked by oxygen consumption by fish and other organisms, organic matter decomposition, phytoplankton blooms, and temperature increase. The objective of the present study was to investigate Na + , Cl -, K + , and ammonia fluxes in silver catfish (Rhamdia quelen) exposed to different dissolved oxygen levels. Juveniles (9 ± 1g) maintained at 6.0 mg.L -1 dissolved oxygen were transferred to four 40 L aquaria with different dissolved oxygen levels (in mg.L -1 ): 6.0, 4.5, 3.5, and 2.5. In another series of experiments, juveniles were acclimated at 6.0 or 2.5 mg.L -1 dissolved oxygen levels, and then placed in two 40 L aquaria with 6.0 mg.L -1 dissolved oxygen. For both series of experiments, 1, 24, 48 or 120 h after transference juveniles were placed in individual chambers of 200 mL (with the same dissolved oxygen levels of their respective aquaria) for 3 h. Water samples were collected for analysis of Na + , Cl -, K + , and ammonia levels. The obtained results allow concluding that exposure to 2.5 mg.L -¹ dissolved oxygen levels promotes loss of ions and lower ammonia excretion in silver catfish juveniles, but these losses are rapidly stabilized for Na + and Cl -. Exposure to less hypoxic levels also changes ion fluxes and ammonia excretion, but there is no clear relationship between both parameters in this species. Therefore, silver catfish osmoregulation seems to be affected when this species is transferred from normoxic to hypoxic waters and vice-versa.Baixos níveis de oxigênio dissolvido na água (hipóxia) podem ser causados pelo consumo de oxygênio por peixes e outros organismos, decomposição de matéria orgânica, "blooms" de fitoplâncton e aumento de temperatura. O objetivo do presente estudo foi analisar os fluxos de Na + , Cl -, K + e amônia em jundiás (Rhamdia quelen) expostos a diferentes níveis de oxigênio dissolvido. Juvenis (9 ± 1g) mantidos em 6,0 mg.L -1 oxigênio dissolvido foram transferidos para quatro aquários de 40 L com diferentes níveis de oxigênio dissolvido (em mg.L -1 ): 6,0; 4,5; 3,5 e 2,5. Em outra série de experimentos, juvenis foram aclimatados a 6,0 ou 2,5 mg.L -1 oxigênio dissolvido e então colocados em dois aquários de 40 L com 6,0 mg.L -1 oxigênio dissolvido. Para ambas séries de experimentos, 1, 24, 48 ou 120 h depois da transferência os juvenis foram colocados em câmaras individuais de 200 mL (com os mesmos níveis de oxigênio dissolvido dos seus respectivos aquários) por 3 h. Amostras de água foram coletadas para análise dos níveis de Na + , Cl -, K + e amônia. Os resultados obtidos permitem concluir que a exposição a 2,5 mg.L -1 oxigênio dissolvido promove uma perda de íons nos juvenis de jundiá, mas estas perdas são estabilizadas rapidamente no caso de Na + e Cl -. A exposição a níveis não tão hipóxicos também altera os fluxos iônicos, mas não há nenhuma relação clara entre ambos parâmetros. Portanto, a osmorregulação do jundiá parece ser afetada quando exemplares são transferidos de águas normóxicas to hipóxicas e vice-ve...

The aquatic environment presents daily and/or seasonal variations in dissolved oxygen (DO) levels. Piava faces different DO levels in the water due to its distributional characteristics. The goal of this study was to describe the effects of low DO levels on plasma ion, biochemical and oxidative variables in piava juveniles. Fish were exposed to different DO levels, including 1.0, 2.0, 3.0, 4.0 and 5.0 mg L-1 of DO for 96 h, after which blood and tissue samples (liver, kidney, gill and muscle) were collected. The decrease in DO levels decreased plasma Na + , Cl-, K + and NH 3 levels as well as protein and glycogen levels in the liver, kidney and muscle; increased Na + /K +-ATPase activity in the gills and kidney as well as glucose and ammonia levels in the liver, kidney and muscle; and increased lactate levels in the kidney and muscle. Thiobarbituric acid-reacting substances, catalase and non-protein thiol levels decreased in the tissues of piavas exposed to low DO levels. It is concluded that piava can apparently cope with hypoxic conditions; however, low DO levels are a stressor, and the tolerance of piava to hypoxia involves iono-regulatory, metabolic and oxidative adjustments.