Abstract
Respirometry is frequently used to estimate metabolic rates and examine organismal responses to environmental change. Although a range of methodologies exists, it remains unclear whether differences in chamber design and exercise (type and duration) produce comparable results within individuals and whether the most appropriate method differs across taxa. We used a repeated-measures design to compare estimates of maximal and standard metabolic rates (MMR and SMR) in four coral reef fish species using the following three methods: (i) prolonged swimming in a traditional swimming respirometer; (ii) short-duration exhaustive chase with air exposure followed by resting respirometry; and (iii) short-duration exhaustive swimming in a circular chamber. We chose species that are steady/prolonged swimmers, using either a body-caudal fin or a median-paired fin swimming mode during routine swimming. Individual MMR estimates differed significantly depending on the method used. Swimming respirometry consistently provided the best (i.e. highest) estimate of MMR in all four species irrespective of swimming mode. Both short-duration protocols (exhaustive chase and swimming in a circular chamber) produced similar MMR estimates, which were up to 38% lower than those obtained during prolonged swimming. Furthermore, underestimates were not consistent across swimming modes or species, indicating that a general correction factor cannot be used. However, SMR estimates (upon recovery from both of the exhausting swimming methods) were consistent across both short-duration methods. Given the increasing use of metabolic data to assess organismal responses to environmental stressors, we recommend carefully considering respirometry protocols before experimentation. Specifically, results should not readily be compared across methods; discrepancies could result in misinterpretation of MMR and aerobic scope.
Highlights
The growing fields of ecological and conservation physiology (Wikelski and Cooke, 2006; Cooke et al, 2013) aim to understand the mechanisms underpinning the behaviour and fitness of organisms in changing environments
The aims of this study were as follows: (i) to compare metabolic rate estimates obtained using three common methods, i.e. a prolonged swim trial using a swimming respirometer, a short-duration exhaustive chase protocol with air exposure followed by resting respirometry, and a short-duration exhaustive swimming trial in a circular chamber followed by resting respirometry; and (ii) to determine whether metabolic rate estimates obtained with each method vary among four fish species, all of which are prolonged swimmers but exhibit different swimming modes (BCF and median–paired fin (MPF))
The value of MMRSwim was consistently higher than MMRChase (z = 2.95, P < 0.01) and MMRCircle (z = 4.79, P < 0.001) for all species (BCF and MPF swimmers; Fig. 3A)
Summary
The growing fields of ecological and conservation physiology (Wikelski and Cooke, 2006; Cooke et al, 2013) aim to understand the mechanisms underpinning the behaviour and fitness of organisms in changing environments. Respirometry is a tool commonly used to measure oxygen consumption rates ( MO2 ) and estimate the metabolic performance of an organism at rest, during exposure to stressors or while performing different locomotory activities. Maximal metabolic rate (MMR) is the maximal amount of energy that can be metabolized aerobically by an organism and can be estimated by measuring an organism’s MO2 during or immediately after exhaustive exercise ( MO2Max; Norin and Clark, 2016). Standard metabolic rate (SMR) is the minimal amount of energy required for maintenance and is estimated by measuring MO2 in a post-absorptive, resting state ( MO2Min; Nelson and Chabot, 2011; Chabot et al, 2016). The difference between MO2Max and MO2Min is the absolute or total scope for aerobic activity (aerobic scope; AS), which is, in essence, the capacity for aerobic metabolism, in excess of basic maintenance costs, for activities essential to support biological fitness, such as swimming, feeding and reproduction (Brett, 1964). Respirometry can provide essential information about the metabolic performance of an organism and is rapidly becoming more widely used (Clark et al, 2013; Norin and Clark, 2016)
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