Influence of a Thermal Effluent on Movement of Brown Bullhead (Ictalurus nebulosus) as Determined by Ultrasonic Tracking

Brown bullhead (Ictalurus nebulosus) were captured between July 31 and August 23, 1973, both within the influence of a thermal discharge in Lake Ontario and at a control site. Ten fish were marked with ultrasonic transmitters and continuously tracked for periods of 29–344 min in the influence of the discharge. Five bullheads from the control site were similarly treated and monitored.Fish caught and released in the discharge area remained within 50–700 m of the release point, turned frequently and sharply, and swam at an average speed of 30 cm/s. Control fish moved along shore, average speed 48 cm/s, in straighter paths up to 4200 m from the release point. On the morning following release, only 20% of fish released in the discharge were still there, indicating that association with the plume was of short duration. The study indicates that the thermal discharge can interrupt or alter normal movement patterns of brown bullhead.