Measuring maximum oxygen uptake with an incremental swimming test and by chasing rainbow trout to exhaustion inside a respirometry chamber yield the same results.

We hypothesized that oxygen uptake (ṀO2 ) measured with a novel protocol of chasing rainbow trout Oncorhynchus mykiss to exhaustion inside a static respirometer while simultaneously monitoring ṀO2 (ṀO2chase ) would generate the same and repeatable peak value as when peak active ṀO2 (ṀO2active ) is measured in a critical swimming speed protocol. To reliably determine peak ṀO2chase , and the peak during recovery of ṀO2 following a conventional chase protocol outside the respirometer (ṀO2rec ), we applied an iterative algorithm and a minimum sampling window duration (i.e., 1 min based on an analysis of the variance in background and exercise ṀO2 ) to account for ṀO2 dynamics. In support of our hypothesis, peak ṀO2active (707 ± 33 mg O2 h-1  kg-1 ) and peak ṀO2chase (663 ± 43 mg O2 h-1  kg-1 ) were similar (P = 0.49) and repeatable (Pearson's and Spearman's correlation test; r ≥ 0.77; P < 0.05) when measured in the same fish. Therefore, we conclude that estimates of ṀO2max can be independent of whether a fish is exhaustively chased inside a respirometer or swum to fatigue in a swim tunnel, provided ṀO2 is analyzed with an iterative algorithm and a minimum but reliable sampling window. The importance of using this analytical approach was illustrated by peak ṀO2chase being 23% higher (P < 0.05) when compared to a conventional sequential interval regression analysis, while using the conventional chase protocol (1-min window) outside the respirometer increased this difference to 31% (P < 0.01). Moreover, because peak ṀO2chase was 18% higher (P < 0.05) than peak ṀO2rec , chasing a fish inside a static respirometer may be a better protocol for obtaining maximum ṀO2 . This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.