Abstract
Wild harbour porpoises (Phocoena phocoena) mainly forage during the night and, because they rely on echolocation to detect their prey, this is also when they are most acoustically active. It has been hypothesised that this activity pattern is a response to the diel behaviour of their major prey species. To test this hypothesis, we monitored the acoustic activity of two captive harbour porpoises held in a net pen continuously during a full year and fed by their human keepers during daylight hours, thus removing the influence of prey activity. The porpoises were exposed to similar temperature and ambient light conditions as free-ranging animals living in the same region. Throughout the year, there was a pronounced diel pattern in acoustic activity of the porpoises, with significantly greater activity at night, and a clear peak around sunrise and sunset throughout the year. Clicking activity was not dependent on lunar illumination or water level. Because the porpoises in the pen are fed and trained during daylight hours, the results indicate that factors other than fish behaviour are strongly influencing the diel clicking behaviour pattern of the species.
Highlights
Wild harbour porpoises (Phocoena phocoena) mainly forage during the night and, because they rely on echolocation to detect their prey, this is when they are most acoustically active
The acoustic activity patterns of captive harbour porpoises are investigated over a full year to understand which extrinsic cues are triggering their daily clicking behaviour
The average clicking activity was considerably higher during the night than during daylight hours throughout the year (Figs. 2A and 3)
Summary
Wild harbour porpoises (Phocoena phocoena) mainly forage during the night and, because they rely on echolocation to detect their prey, this is when they are most acoustically active. It has been hypothesised that this activity pattern is a response to the diel behaviour of their major prey species To test this hypothesis, we monitored the acoustic activity of two captive harbour porpoises held in a net pen continuously during a full year and fed by their human keepers during daylight hours, removing the influence of prey activity. Because the porpoises in the pen are fed and trained during daylight hours, the results indicate that factors other than fish behaviour are strongly influencing the diel clicking behaviour pattern of the species. The acoustic activity patterns of captive harbour porpoises are investigated over a full year to understand which extrinsic cues are triggering their daily clicking behaviour. When compared to wild porpoises, cues generated by prey behaviour can be ruled out, but a strong additional cue, feeding events in the Scientific Reports | (2020) 10:14876
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