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 ((Formula presented.) 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 (Formula presented.) O2 determinations, producing a histogram of the r2 values and a plot of r2 as a function of (Formula presented.) 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 r2s.