Valid oxygen uptake measurements: using high r 2 values with good intentions can bias upward the determination of standard metabolic rate.

This analysis shows good intentions in the selection of valid and precise oxygen uptake ( M ˙ O ₂ ) measurements by retaining only slopes of declining dissolved oxygen level in a respirometer that have very high values of the coefficient of determination, r ² , are not always successful at excluding nonlinear slopes. Much worse, by potentially removing linear slopes that have low r ² only because of a low signal-to-noise ratio, this procedure can overestimate the calculation of standard metabolic rate (SMR) of the fish. To remedy this possibility, a few simple diagnostic tools are demonstrated to assess the appropriateness of a given minimum acceptable r ² , such as calculating the proportion of rejected M ˙ O ₂ determinations, producing a histogram of the r ² values and a plot of r ² as a function of M ˙ O ₂ . The authors offer solutions for cases when many linear slopes have low r ² . The least satisfactory but easiest to implement is lowering the minimum acceptable r ² . More satisfactory solutions involve processing (smoothing) the raw signal of dissolved oxygen as a function of time to improve the signal-to-noise ratio and the r ² s.
(© 2020 Her Majesty the Queen in Right of Canada. Journal of Fish Biology published by John Wiley & Sons Ltd on behalf of Fisheries Society of the British Isles.)