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The effects of steady-state, aerobic swimming exercise upon the cardiovascular system of rainbow trout (Salmo gairdneri) are discussed. When these fish are forced to swim at 80% of their critical velocity, blood flow is redistributed in the systemic circulation to favour working muscles, at the expense of decreased flow to nonmuscle structures. Given oxygen uptake and cardiac output data, combined with blood flow distribution patterns during exercise, it is calculated that the working muscles can account for nearly all the measured increase in total oxygen uptake at this level of exercise. The possible underlying mechanisms for systemic blood flow control and the increase in gas exchange efficiency across the gills of exercising rainbow trout also are examined.

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A new technique for the detection in situ of oxygen-sensitive chemoreceptors in fish is described. By studying the hypoxic responses of the rainbow trout, Salmo gairdneri, at least two loci of respiratory control were found to be operating. One component initiated the hypoxic bradycardia and its rapid release, a second was responsible for ventilatory responses. The anterodorsal region of the first branchial arches has been established as a peripheral oxygen-sensitive chemoreceptor site, mediating bradycardia, without affecting either the amplitude or frequency of breathing. The study does not preclude the existence of oxygen receptors in other areas of the fish.