Animals get the energy to sustain life by oxidizing reduced carbon compounds in food, therefore metabolic rate can be measured experimentally by determining an animal's rate of oxygen consumption (mg O2/kg body weight/hr). Fish can be placed in underwater chambers where their uptake of O2 can be accurately recorded, moreover the temperature can be varied and the fish can be made to swim. On this type of experimentation, rests our knowledge of how the metabolic rate of fish varies. Fish are poikilotherms. They don't regulate their body temperature, thus have variable metabolic rates dependent on the temperature of their body at the time. Basal metabolic rate (BMR) in mammals is the rate of oxygen consumption at a resting state. The concept of BMR is not appropriate for fish because the metabolic rate in a resting fish will vary depending on the ambient temperature. Standard Metabolic Rate (SMR) is the metabolic benchmark in fish. It is the metabolic rate of a resting fish at a specified temperature in the middle of its normal range. Therefore a trout's SMR15 would be the resting metabolic rate of a trout at 15oC. There is natural law called Q10 = 2: chemical reactions tend to double with every 10o C increase in temperature of the reactants. In poikilotherms, this is translated into biochemical events and in turn into metabolism. The metabolic rates of fish roughly double with every 10o
C increase in temperature,
except at the extreme ends of their temperature tolerance.
Over a broad range of dissolved oxygen (DO) concentrations at a given temperature and activity level, a fish has a constant metabolic rate. There is a critical point of DO, however, below which a fish cannot extract enough oxygen from the water to support its metabolic rate. At this point, a fish begins to accumulate oxygen debt. That is, it must generate some of its ATP from anaerobic sources (glycolysis) and lactate, which it cannot further oxidize, accumulates in the tissues (See Ch. IVB). A fish cannot live indefinitely below the critical point. At some point, as DO continues to fall, the fish is unable to sustain life anaerobicly and death from asphyxiation occurs.
In all animals, a higher rate of physical activity (exercise) oxidizes more calories. In fish, whose metabolic rate is affected by the oxygen content of the water, the oxygen effect interacts with the activity effect. As the rate of activity increases, the fish needs more oxygen in the water to sustain aerobic metabolism, therefore the critical point shifts to the right.
At any given level of temperature, oxygen and activity, two different species may have different metabolic rates. For example, a carp and a trout have different hemoglobin oxygen dissociation curves (See Ch. IIE) and different tissue tolerances for hypoxia. This allows the carp to tolerate lower DO conditions without going into oxygen debt (lower critical point) and trout to sustain a higher rate of activity (higher critical point). It is an evolutionary trade off that adapts species to different habitats and behaviors.
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