Acoustically adept species in the marine environment have to contend with complex and highly variable soundscapes. In the ocean today, sounds from human sources contribute substantially to the underwater acoustic environment. We used a 4-element hydrophone array in Glacier Bay National Park to (1) identify primary drivers of ambient sound in this region, (2) investigate whether humpback whales Megaptera novaeangliae exhibit a Lombard response in response to ambient noise, and (3) investigate whether humpback whales adjust their calling activity in response to naturally occurring and vessel-generated sounds. We found that cruise ships and tour boats, roaring harbor seals Phoca vitulina, and weather events were primary drivers of ambient sound levels, and that they varied both seasonally and diurnally. As ambient sound levels increased, humpback whales responded by increasing the source levels of their calls (non-song vocalizations) by 0.81 dB (95% CI = 0.79-0.90) for every 1 dB increase in ambient sound. There was no evidence that the magnitude of the observed response differed between natural and man-made sounds. We also found that the probability of a humpback whale calling in the survey area decreased by 9% for every 1 dB increase in ambient sound. Controlling for ambient sound levels, the probability of a humpback whale calling in the survey area was 31-45% lower when vessel noise contributed to the soundscape than when only natural sounds were present.

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