Valid oxygen uptake measurements: using high r2 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 ˙ O2 ) measurements by retaining only slopes of declining dissolved oxygen level in a respirometer that have very high values of the coefficient of determination, r2 , are not always successful at excluding nonlinear slopes. Much worse, by potentially removing linear slopes that have low r2 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 r2 , such as calculating the proportion of rejected M ˙ O2 determinations, producing a histogram of the r2 values and a plot of r2 as a function of M ˙ O2 . The authors offer solutions for cases when many linear slopes have low r2 . The least satisfactory but easiest to implement is lowering the minimum acceptable r2 . 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 r2 s.