The reaction of bottlenose dolphins to brief, steady-state startle sounds in the wild

Our work on the startle reflex in cetaceans, which we first demonstrated in bottlenose dolphins in collaboration with the Nachtigall laboratory, has highlighted the potential for behavioral change at comparatively low noise exposure levels. Here, we studied the effects of startle sounds on dolphin movement in the wild. We exposed groups of bottlenose dolphins to five brief (200 ms duration) startle sounds (10–20 kHz peak frequency) at irregular intervals over a 1-min period (max. received level 147 dB re 1 μPa) and compared their subsequent movement to the pre-exposure period and to control periods without noise exposure. During exposure, dolphins changed their heading and increased their swim speed in an avoidance response that lasted up to 4 min, resulting in a distance of 500–800 m from the source at the end of this period. In control periods, no change of behavior was observed. Our results demonstrate a clear movement response to a relatively minor noise dose. While a short-term displacement will have little impact on animals and can be used to manage marine mammal occurrence in areas that could otherwise lead to injury, further studies are needed to understand their broader effects on marine mammals.