Response of rainbow trout (Oncorhynchus mykiss) to a graded hypoxia following repeated intermittent exposure to chloramine-T

The physiological effects of repeated exposure to 9 mg l(-1) chloramine-T, a common aquaculture disinfectant for rainbow trout, were examined using a graded hypoxic challenge. Using an extracorporeal circulation, continuous measurements of blood PO2, PCO2 and pH were made and correlated with decreasing water PO2. Ventilation amplitude and frequency were also monitored. Following the graded hypoxic challenge, the gills were removed and processed for microscopy for morphometric measurements and the determination of the number of branchial mucous cells. Fish treated with chloramine-T exhibited a higher arterial PO2 during hypoxia between 10.1 and 11.2 kPa when compared with untreated controls; there were no differences in arterial PCO2 or pH between the two groups. Chloramine-T-treated fish had an elevated pre-hypoxic ventilation frequency as compared with the controls. However, under the graded hypoxia, control fish elevated their ventilation frequency, whereas chloramine-T-treated fish did not. Both chloramine-T-treated and control fish increased their ventilation amplitude during the graded hypoxia and there were no differences between control and chloramine-T-treated fish. The fish treated with chloramine-T had a reduced thickness of the gill epithelial blood-to-water diffusion barrier but higher numbers of mucous cells as compared with controls. We suggest that although there was a mucous cell hyperplasia in response to repeated chloramine-T exposure, the thinning of the lamellar epithelium was sufficient to offset any diffusive limitations, thus ensuring that gas exchange was not adversely affected. (C) 1998 Elsevier Science B.V. All rights reserved.