What male humpback whale song chorusing can and cannot tell us about their ecology: strengths and limitations of passive acoustic monitoring of a vocally active baleen whale
Passive acoustic monitoring (PAM) with autonomous bottom-moored recorders is widely used to study cetacean occurrence, distribution, and behaviors, as it is not subject to factors that limit other traditional observation methods (e.g. vessel, land and aerial-based surveys) such as inclement weather, sighting conditions, the remoteness of study sites, etc. PAM projects range widely from presence/absence studies to addressing questions about noise exposure. On the Hawaiian breeding ground, an estimated 12,000 humpback whales migrate annually from their high-latitude feeding grounds in Alaska to mate and give birth. While on the breeding grounds males produce an elaborate acoustic display known as song. Multiple males usually sing concurrently resulting in an ongoing chorus of song throughout the season. This song, primarily produced at peak frequencies under 2 kHz, can be captured using PAM, thus providing a powerful tool to monitor the humpback whale population and to study the actual song itself and its ecological importance. Calculating root-mean-square sound pressure levels (RMS SPLs in dB re 1 μPa) in one-octave bands using data from long-term PAM recordings made off Maui, we can compare the low frequency acoustic energy (0-1.56 kHz) produced by singing males across time and sites. Over the season, energy levels start increasing in December, peak in February and March, before dropping in late March through April. This mirrors the whales’ migratory pattern and indicates that male chorusing can be used as a proxy for relative whale abundance. The uses for PAM can be varied, including monitoring the relative abundance at the same site over time and comparing occurrence among multiple recording locations to understand spatial patterns of habitat use. However, on the breeding grounds and in contrast to males, females and mother-calf pairs have been shown to be acoustically cryptic and any PAM study on humpback whales will predominantly capture singing males. Furthermore, some observed spatial and temporal acoustic patterns can be ambiguous. The relationship between recordings from singing males and whale abundance is still poorly understood and previous attempts to correlate acoustic recordings with abundance have proved challenging. We propose that linking acoustics with other survey methods, particularly land and vessel-based observations, will greatly reduce the ambiguity typical of each individual method. Continued efforts to relate trends in acoustic levels to actual whale numbers and to understand the influence of recording strategies are planned.