Sex Differences in the Heart Rate Variability Spectrum of Free-Swimming Atlantic Salmon (Salmo salar L.) during the Spawning Season

Heart rates were telemetered from male and female adult salmon (Salmo salar L.) when they were ascending a river to spawn. In this specific period, males had higher heart rates than females, reflecting the higher activity levels and higher metabolic rates of the males. Analysis of the beat-to-beat intervals (using the heart rate variability spectrum) revealed a single spectral peak that differed between sexes, with males displaying a spectral peak at lower frequencies than females even when the heart rate was the same. The dual spectral peak observed in higher vertebrates and in some other studies in fish was not found. It is suggested that the single spectral peak is related to the activity of the blood pressure control loop. In this framework, the gender spectral differences could be explained by a differential vascular reactivity related to different sex steroid concentrations in plasma. Spectral analysis of beat-to-beat intervals is extensively used to understand short-term control of heart rate in mammals; this study indicates that it can also be applied to free-living fish to understand neural cardiovascular regulation.