| Summary: | Recreational boating is increasingly popular, and noise from small boats may have a strong impact on the health of cetaceans. Although the animals’ reactions to boat noise have been studied, this has not previously been done in controlled settings. Here, we investigated how noise from a small boat influenced harbor porpoise ( Phocoena phocoena ) behavior using a drone. We collected porpoise responses when passing the animals at 10 and 20 knots. The animals already started changing behavior when the boat was <250m away. When the boat approached, the porpoises speeded up, and moved away from boat in generally but not consistently. However, we did not find porpoises diving longer during exposure. Our study showed that drones can be used for monitoring behavioural changes in free-ranging marine mammals, and the results give us important knowledge about how porpoise energetics and fitness are influenced by boat noise.
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