Humpback and Minke Whales Increase the Intensity of Their Calls in Increased Background Noise From Natural Sources

Humpback whale song and minke whale boing calls are commonly recorded on the U.S. Navy’s Pacific Missile Range Facility (PMRF) off the Hawaiian Islands throughout the winter. The Navy uses this area for training, which periodically includes sonar transmission. Marine mammal behavioral changes due to sonar have been the subject of much recent research, but marine mammals also change their behavior due to natural environmental fluctuations. Calling minke and humpback whales were detected, localized, and tracked on PMRF over 5 years (2012-2017). Over 1,000 minke tracks and over 100 humpback tracks were verified. These tracks consisted of ~10⁴ minke whale boing calls and ~10⁵ humpback whale song units. We measured the source levels of humpback whale song units and minke whale boing calls on PMRF during times when there were no Navy training activities. Both humpback and minke whales increased the source levels of their calls during times of higher background noise. The variance of minke boing call source levels decreased as noise levels increased, suggesting that minke whales become more precise with their call intensity in higher background noise levels. However, neither species fully compensated for the increase in background noise, so their communication range decreased in increased noise conditions. These behavioral changes due to natural events will help us to contextualize behavioral changes due to anthropogenic noise sources. Marine mammal populations are regularly assessed, but many cetacean species, such as small, solitary species like the minke whale, are difficult to detect visually, so much is unknown about their population sizes. Passive acoustic monitoring has been suggested as a way to estimate population size if a conversion can be made from number of calls to number of whales. Source level, which is needed in this conversion, is often assumed to be a constant value, but this research shows that source level as a function of background noise level may be more accurate.