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

The Fick principle has often been used to estimate the volume flow of water per unit time over the gills of fishes. Application of the Fick principle requires accurate measurement of mean oxygen tensions in water entering and leaving the gills and of total oxygen uptake. Cannulae of various types have frequently been used to sample water entering and leaving the gills (van Dam, 1938; Holeton & Randall, 1967a, b; Randall, Holeton & Stevens, 1967; Saunders, 1961,1962; Stevens & Randall, 1967 a, b). Recent studies have indicated that these cannulation techniques sometimes provide poor estimates of mean expired oxygen tensions (Davis & Watters, 1970; Garey, 1967). Procedures of this type therefore could lead to inaccurate estimates of volume flow of water over the gills. Hence it was necessary to devise another method of measuring ventilation volume in this study. Variations in ventilation volume influence gas exchange across the gills. Hughes (1966) and Saunders (1961) discussed the effects of high gill water flow, pointing out that anatomical deadspace increases under these conditions since portions of the ventilatory flow are shunted past the tips of adjacent gill filaments. It was a further objective of this study to see how the velocity of water flow over the gills influenced a fish's ability to exchange gases with the water.