The first direct measurements of ventilatory flow and oxygen utilization after exhaustive exercise and natural feeding in a teleost fish, Oncorhynchus mykiss

A new “less invasive” device incorporating an ultrasonic flow probe and a divided chamber, but no stitching of membranes to the fish, was employed to make the first direct measurements of ventilatory flow rate (V̇w) and % O 2 utilization (%U) in juvenile rainbow trout (37 g, 8ºC) after exhaustive exercise (10-min chasing) and natural voluntary feeding (2.72% body mass ration). Under resting conditions, the allometrically scaled V̇w (300 ml kg −1 min −1 for a 37-g trout = 147 ml kg −1 min −1 for a 236-g trout exhibiting the same mass-specific O 2 consumption rate, ṀO 2 ) and the convection requirement for O 2 (CR = 4.13 L mmol −1 ) were considerably lower, and the %U (67%) was considerably higher than in previous studies using surgically attached masks or the Fick principle. After exhaustive exercise, V̇w and ṀO 2 approximately doubled whereas frequency (fr) and %U barely changed, so increased ventilatory stroke volume (Vsv) was the most important contributor to increased ṀO 2 . CR declined slightly. Values gradually returned to control conditions after 2–3 h. After voluntary feeding, short-term increases in V̇w, Vsv and ṀO 2 were comparable to those after exercise, and fr again did not change. However, %U increased so CR declined even more. The initial peaks in V̇w, Vsv and ṀO 2, similar to those after exercise, were likely influenced by the excitement and exercise component of natural feeding. However, in contrast to post-exercise fish, post-prandial fish exhibited second peaks in these same parameters at 1–3 h after feeding, and %U increased further, surpassing 85%, reflecting the true “specific dynamic action” response. We conclude that respiration in trout is much more efficient than previously believed.