Importance of electrode positioning in biotelemetry studies estimating muscle activity in fish

Red and white axial muscle activity of adult Atlantic salmon Salmo salar was examined using conventional electromyography (EMGx) and activity radio‐transmitters (EMGi) at 0·5 and 0.7 body lengths (L) along the body of the fish. Critical swimming trials were conducted and maximum sustainable speeds (Ucrit) were unaffected by the presence of electrodes, being 1·51 ± 21 m s−1 (3.33 ± 0.34 L s−1) (n=44). Regardless of longitudinal position of the electrodes within the musculature, both EMGx s and EMGi s indicated increasing red muscle activity with increasing swimming speed, whereas white muscle fibres were recruited only at speeds > 86±5% Ucrit. Telemetered EMGi signals indicated that muscle activity varied significantly for electrodes implanted at different longitudinal positions along the fish (P < 0·001). These results suggest that electrode placement is an important influence affecting the signals obtained from radio transmitters that estimate activity and location should be standardized within biotelemetry studies to allow accurate and consistent comparisons of activity between individuals and species. Optimal location for electrode placement was determined to be in the red muscle, towards the tail of the fish (0·7 L).