Measurement and relevance of maximum metabolic rate in fishes

Maximum (aerobic) metabolic rate (MMR) is defined here as the maximum rate of oxygen consumption (Ṁ_2max) that a fish can achieve at a given temperature under any ecologically relevant circumstance. Different techniques exist for eliciting MMR of fishes, of which swim‐flume respirometry (critical swimming speed tests and burst‐swimming protocols) and exhaustive chases are the most common. Available data suggest that the most suitable method for eliciting MMR varies with species and ecotype, and depends on the propensity of the fish to sustain swimming for extended durations as well as its capacity to simultaneously exercise and digest food. MMR varies substantially (> 10 fold) between species with different lifestyles (i.e. interspecific variation), and to a lesser extent (< three‐fold) between individuals of the same species (i.e. intraspecific variation). MMR often changes allometrically with body size and is modulated by several environmental factors, including temperature and oxygen availability. Due to the significance of MMR in determining aerobic scope, interest in measuring this trait has spread across disciplines in attempts to predict effects of climate change on fish populations. Here, various techniques used to elicit and measure MMR in different fish species with contrasting lifestyles are outlined and the relevance of MMR to the ecology, fitness and climate change resilience of fishes is discussed.