Figure 6.

An analysis of the oxygen cost of swimming to the critical swimming speed (U_crit) by incorporating the excess post-exercise oxygen consumption (EPOC). Data were adapted from Lee et al. (2003b), where details of the methodology are presented. It is assumed during the later stages of swimming that anaerobic swimming results in the measured oxygen consumption (Mo₂; solid line) being an underestimate of the true oxygen constant at these higher swimming speeds. It was further assumed that the EPOC measured during recovery after U_crit (solid line) reflected the deferred oxygen costs of swimming and that the overall cost of locomotion through water is exponentially related to swimming speed. To allow a continuous plot of Mo₂ measured during the swim test and during the recovery period (solid line), data for Gates Creek sockeye salmon and Chehallis coho salmon (taken from figure 5) were transformed by plotting Mo₂ against the experimental time (this is possible because the speed increments occur at fixed time-intervals). EPOC (indicated numerically) was then derived from the integral of the Mo₂ measured post-U_crit and bounded by routine Mo₂. The integrated EPOC could then be applied to the measured Mo₂ during the swim test (through iterative mathematical modelling) to generate the derived oxygen cost of swimming (broken line) such that the integrated EPOC matched the numerical difference (as indicated) between the derived and measured Mo₂. The net result is an estimate of the extent to which direct measurements on Mo₂ during swimming by themselves underestimate the oxygen cost of swimming at U_crit.