Perhaps, the paper I reference you below (which uses "ultrasonic telemetry" in fishes, can be of interest for your research problem.
"The ultrasonic telemetry of cardiac rhythms of wild brown trout (Salmo trutta L.) as an indicator of bio-energetics and behaviour"; by: I. G. Priede† andA. H. Young.
Journal of Fish Biology, Volume 10, Issue 4, pages 299–318, April 1977