The effect of body mass and temperature on the heart rate, stroke volume, and cardiac output of larvae of the rainbow trout, Oncorhynchus mykiss.

The goal of this experiment was to determine whether the type of tight supply-and-demand relationship between cardiac output and metabolic demand for O2 seen in juvenile and adult fish applies during larval development. To this end, we looked at how the heart rate, stroke volume, and cardiac output of rainbow trout (Oncorhynchus mykiss) larvae varied in response to changes in tissue mass and incubation temperature. Previous studies have shown that both factors have a profound influence on metabolic rate. Heart rate and stroke volume were estimated using videomicroscopic methods and used to calculate cardiac output at five or six approximately evenly spaced intervals between hatch (approximately 15 mg wet tissue mass) and 150 accumulated thermal units (degrees C d) posthatch (approximately 50 mg tissue mass) at 5 degrees, 10 degrees, 12 degrees, and 15 degrees C. Cardiac output (range 0.2-20 microL min(-1)) increased significantly in response to increases in both tissue mass and incubation temperature. The increase in cardiac output with tissue mass reflected significant increases in stroke volume as well as heart rate. Temperature only had a significant effect on heart rate (i.e., stroke volume was unaffected). The rate of increase in cardiac output as a result of tissue growth was significantly faster than the rate at which O2 demand increased (the allometric mass exponent [+/-SE] for cardiac output was 1.78 +/- 0.08; literature values for O2 uptake average approximately 1.0), which suggests that the cardiovascular system was less important as a means of delivering O2 to the tissues in small larvae than it was in larger larvae and in juvenile and adult fish. Direct diffusion appeared to be the primary means of O2 delivery in small larvae and embryos.