Valid oxygen uptake measurements: using high r2 values with good intentions can bias upward the determination of standard metabolic rate.

Abstract

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, 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˙O2 determinations, producing a histogram of the r 2 values and a plot of r 2 as a function of M˙O2. 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 2s.