Water flow and gas exchange at the gills of rainbow trout, Salmo gairdneri.

Abstract

ABSTRACT Ventilation volume was measured directly in rainbow trout using a rubber membrane attached to the mouth which separated inspired and expired water and allowed collection of the latter. Mean ventilation volume at 8·6°C for 18 trout weighing approximately 200 g was 37±1·8 ml/min/fish. Mean ventilation rate and ventilatory stroke volume averaged 74 breaths/min and 0·5 ml/breath respectively. Ventilation volume could be increased nearly sevenfold during moderate, shortterm hypoxia as a result of a large increase in ventilatory stroke volume and a small increase in ventilation rate. The ratio between the flow rates of water and blood through the gills was approximately 10. Percentage utilization of oxygen from inspired water had a mean of 46 ± 1·5% and ranged from 23 to 64%. Artificial perfusion of the gills with water at different flow rates was achieved by tying a tube into the mouth of trout. Perfused fish could not saturate their arterial blood with oxygen at a perfusion rate of 45 ml/min but could do so at rates ranging from 85 to 1200 ml/min. Low arterial tensions at a perfusion rate approximating the mean of fish with oral membranes are probably the result of a poor pattern of water flow over the gills during perfusion. Opercular movements occurred only at perfusion rates below 700 ml/min and increased in frequency as perfusion rate dropped. This ventilatory activity may have resulted from receptors sensitive either to water flow over the gills or to arterial . As perfusion rate went up cardiac output and oxygen uptake increased. These changes were accompanied by a drop in dorsal aortic pressure which reflected vasodilation of the gills and peripheral circulation. This change in the pattern of blood flow through the gills contributed to a 50% increase in oxygen transfer factor across the gills. At the highest perfusion rates there was no apparent impairment of gas exchange even though anatomical deadspace was probably high.