Displacement of migrating bowhead whales by sounds from seismic surveys in shallow waters of the Beaufort Sea

Abstract

Seismic surveys for subsea oil deposits were conducted each summer, 1996–98, mainly in water <20 m deep. Airgun arrays were used, with 6–16 airguns and total volumes 560–1500 cu.in. Low-frequency sound pulses were created at intervals of 8–20 s. Effective source levels for horizontal propagation were lower than nominal source levels, but pulses were often detectable to 50+ km offshore. Westward autumn migration of bowhead whales near and offshore of the exploration area was monitored by aerial surveys flown daily, weather permitting, during the three seasons. Aerial survey data from days with and without airgun operations were compared. Most bowheads avoided the area within 20 km of the operating airguns; bowheads were common there on days without airgun operations. In 1998, numbers sighted 20–30 km away were also significantly reduced during airgun operations. Conversely, sighting rates just beyond the avoidance zone were higher on days with airgun operations. Broadband received levels of airgun pulses at 20 km were typically 120–130 dB re: 1 μPa (rms over pulse duration), lower than those previously demonstrated to cause avoidance by bowheads. Many migrating bowheads 20 to 50+ km offshore were exposed to weaker but presumably detectable pulses. [Work supported by Western Geophysical and BP Exploration (Alaska), Inc.]