THE METABOLIC DEMAND FOR OXYGEN IN FISH, PARTICULARLY SALMONIDS, AND A COMPARISON WITH OTHER VERTEBRATES’
Active metabolic rates of fish show a five-fold difference between species. Maximum oxygen consumption rates reach about 1000_+200 mg O,/kg/hr (700,140 cc O,/kg/hr) which is comparable with. if not greater than, that for most amphibians and reptiles. In the weight range of 3 to 30 g, mammals and birds expend energy at a maximum sustained rate of about IO to 100 times that of active fish like salmon. Small mammals operate on a minimum transport metabolic rate of about 50 to 100 times the routine (transport) rate of salmon. However, estimation of the momentary demand for oxygen during anaerobic, burst performance of a loo-gram salmon approximates the aerobic, transport metabolic rate of a mammal of corresponding weight. The respiratory-circulatory complex of salmon appears to have evolved to meet the demands placed on it during migration; other daily requirements average no more than one-half that of the metabolic demand for upstream migration. In those exceptional species of fish which have evolved deep muscle homeothermy no great increase in oxygen uptake over other fast-swimming species can be expected. Aerial respiration alone, in amphibians and reptiles, was not accompanied by any particular increase in O,-consumption rate over that of fish, despite the liberation from a highly restrictive respiratory medium. It is concluded that among many factors which accompanied the respiratory evolution of vertebrates the coupling of aerial respiration with homeothermy permitted the immense increase in the ability of vertebrates to consume oxygen.