Directional responses of blueback herring to ultrasonic pulses

Abstract

Adult blueback herring (Alosa aestivalis) have been found to be directionally responsive to very high (118 kHz fundamental) pulsed sounds. The experiments were conducted in a pen containing filtered fresh water. Fish responses were videotaped. Fish usually only flinched in response to the onset of continuous wave stimuli. They usually moved immediately away from the source of short repetitive impulse sounds (duration from around 20 cycles to about four ms and less than 174 dB re 1 mPa at the nearest fish). Although a variety of pulse repetition rates were tested, no clear effect of that variable on fish response was apparent within the limits set by the experimental design. Sounds of similar frequency (120 kHz fundamental, 4 ms pulse duration, 5 Hz repetition rate) but over 200 dB re 1 mPa at the nearest fish also produced a behavioral response which sometimes resulted in the fish moving quickly away from the sound source but always included a dramatic and immediate coalescing into a tight school, as is often seen in the presence of a predation threat. Possible implications for a putative coevolution between Alosine shads and marine mammel predators as well as for ultrasound source localization are discussed.