Examples of 4 hour heart rate recordings from two different fish from the control groups.

<p>(A) The fifth recording day after anaesthesia in a fish that received an injection of buprenorphine. (B) The first recording day after anaesthesia in a fish that did not receive an injection of buprenorphine. The white cross in the centre of each cloud indicates the mean heart rate for the 4 hour recording period and the white circle indicates the heart rate variability (HRV).