The reaction of humpback whales to underwater explosions: Orientation, movements, and behavior

In 1992, local fishermen reported unusually high net collision rates by humpback whales in Bull Arm, Trinity Bay, Newfoundland (47° 45′N, 53° 50′W), an area of underwater industrial activity. As part of a study to investigate this phenomenon [see also Ketten et al., this meeting], levels and types of noise—including underwater explosions—were sampled. The location and movement of a small group of humpbacks (71 individuals identified over a 19-day period) resident in Bull Arm were monitored; when possible, behavior of individuals was recorded directly. CTD profiles and bait abundance were also noted. Explosions were of high amplitude and low frequency. Measured at 1 mile from source, levels typically reached 150 dB (re: 1 μPa at 1 m, at 350 Hz). Following explosions, residency time and location of individual humpbacks did not change. When individuals could be observed directly, no behavioral reaction to explosions (sudden dives, abrupt movements) were seen. Although not statistically significant, more animals were sighted and resighting rates were higher in the explosion area than in other parts of the bay. However, two animals recollided with fishing gear—such reports of successive entrapments are rare.