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) chloramin e-T, a common aquaculture disinfectant for rainbow trout, were examine d using a graded hypoxic challenge. Using an extracorporeal circulatio n, continuous measurements of blood PO2, PCO2 and pH were made and cor related with decreasing water PO2. Ventilation amplitude and frequency were also monitored. Following the graded hypoxic challenge, the gill s were removed and processed for microscopy for morphometric measureme nts and the determination of the number of branchial mucous cells. Fis h treated with chloramine-T exhibited a higher arterial PO2 during hyp oxia between 10.1 and 11.2 kPa when compared with untreated controls; there were no differences in arterial PCO2 or pH between the two group s. Chloramine-T-treated fish had an elevated pre-hypoxic ventilation f requency as compared with the controls. However, under the graded hypo xia, control fish elevated their ventilation frequency, whereas chlora mine-T-treated fish did not. Both chloramine-T-treated and control fis h increased their ventilation amplitude during the graded hypoxia and there were no differences between control and chloramine-T-treated fis h. The fish treated with chloramine-T had a reduced thickness of the g ill 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 ex posure, the thinning of the lamellar epithelium was sufficient to offs et any diffusive limitations, thus ensuring that gas exchange was not adversely affected. (C) 1998 Elsevier Science B.V. All rights reserved .