Humpback whales (Megaptera novaeangliae) are a widely distributed baleen whale species, well known for their diverse acoustic behavior. On high-latitude foraging grounds, humpback whales produce a suite of non-song vocalizations (“calls”) in concert with foraging and social behavior. In this dissertation I investigated the role of calls in the acoustic ecology of humpback whales in Southeast Alaska and the potential impacts of vessel noise on humpback whale calling behavior in this region.
By comparing the earliest known acoustic recordings of humpback whales in Southeast Alaska (1970’s) with recordings collected in the 1990’s, 2000’s, and 2010’s, I determined that of the sixteen known humpback whale call types produced in Southeast Alaska, twelve were stable over a 36-year time period; eight call types were present in all four decades and every call type was present in at least three decades. I conclude that repertoire stability at this temporal scale is indicative of multi-generational persistence and confirms that acoustic communication in humpback whales is best described as a coupled system of communication that contains some highly stable call elements in strong contrast to ever-changing song.
I further looked for evidence of shared call types between two allopatric humpback whale populations while on their northern hemisphere foraging grounds in order to test the hypothesis that some calls are fixed within the humpback whale acoustic repertoire. Despite being geographically and genetically discrete populations, humpback whales in Southeast Alaska (North Pacific Ocean) share at least five call types – known as droplets, growls, teepees, swops, and whups – with humpback whales in Massachusetts Bay (North Atlantic Ocean). This study is the first to identify commonly produced call types shared by allopatric populations and provides additional evidence that some call types may be innate within the humpback whale repertoire.
One call type, the “feeding call” was noticeably absent from the North Atlantic repertoire. The feeding call is a highly stereotyped tonal call with a peak frequency of approximately 500 Hz that to-date has previously been documented only among groups (>2 individuals) of Alaskan humpback whales engaged in synchronized foraging events while feeding on Pacific herring (Clupea pallasii). I described feeding call use by solitary humpback whales throughout Southeast Alaska over a twenty-year time period and suggest that a primary function of this call is prey manipulation. These observations demonstrate that the use of feeding calls is not exclusively linked to group coordination, and may not be exclusively linked to social recruitment.
In order to get a baseline understanding of call characteristics in Southeast Alaska I measured humpback whale sound source levels in Glacier Bay National Park and Preserve (GBNPP). The source levels of 426 humpback whale calls in 4 vocal classes were estimated using a four-element planar array deployed in the Beardslee Island Complex of GBNPP. There was no significant difference in source levels between humpback whale vocal classes. The mean call source level was 137 dBRMS re 1 µPa @ 1 m in the bandwidth of the call (range 113 – 157 dBRMS re 1 µPa @ 1 m), where bandwidth is defined as the frequency range from the lowest to the highest frequency component of the call. This is quieter than reported source levels for both calls on migratory corridors and song produced on breeding grounds.
Finally, I documented temporal occurrence patterns and levels of ambient sound in GBNPP in order to investigate and compare humpback whale vocal responses to natural sounds and vessel sounds. Cruise ships and tour boats, roaring harbor seals (Phoca vitulina), and weather events were primary drivers of ambient sound levels; sound sources varied seasonally and diurnally. As ambient sound levels in the bandwidth of humpback whale calls rose, humpback whales responded by increasing the source levels of their calls 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 anthropogenic sources. The probability of a humpback whale calling in the survey area decreased by 9-10% for every 1 dB increase in ambient sound. After controlling for ambient sound levels, the probability of a humpback whale calling in the survey area was 25-30% lower when vessel noise contributed to the soundscape than when only natural sounds were present. Although natural and anthropogenic sound sources contribute similar amounts of acoustic energy to the soundscape, humpback whale responses to these two sources were not uniform; the magnitude of the Lombard effect was equal between natural and manmade sounds, however the probability of a humpback whale calling was significantly lower when anthropogenic sound contributed to the environment. Future research investigating humpback whales’ response to noise, and the broader effects of noise on vital life functions are merited.
This work expands what is known about humpback whale calling behavior in Southeast Alaska, and more broadly for this species everywhere. This work also demonstrates the importance of documenting calling behavior in the context of both the acoustic habitat as well as species’ broader acoustic ecology.