Seal responses to airgun sounds during summer seismic surveys in the Alaskan Beaufort Sea

Numbers, sighting distances, and behavior of seals were studied during a ne arshore seismic program off northern Alaska in July-September 1996. We obse rved from the seismic vessel for 885.6 h, including all periods (day and ni ght) when airguns operated and many periods without airguns. Of 422 seals s een, 421 were seen in daylight; 91.8% were ringed seals, 7.3% were bearded seals, and 0.9% were spotted seals. About 79% were first seen within 250 m of the seismic boat, and sighting rate declined rapidly at lateral distance s > 50 m. During daylight, seals were seen at nearly identical rates (0.60- 0.63/h) during periods with no airguns firing, one airgun, and a "full-arra y" of 8-11 120-in(3) airguns. However, seals tended to be farther away (P < 0.0001) during full-array seismic. There was partial avoidance of the zone < 150 rn from the boat during full-array seismic, but seals apparently did not move much beyond 250 m. "Swimming away" was more common during full-ar ray than no-airgun periods, but relative frequencies of five behaviors did not differ significantly among distance categories. Airgun operations were interrupted 112 times when seals were sighted within safety radii (150-250 m). The National Marine Fisheries Service specified these radii in the Inci dental Harassment Authorization issued for the project; they are based on a 190 dB re I mu Pa (rms) criterion for broadband received level. Methods fo r estimating numbers of seals potentially affected by the seismic program a re described, and effectiveness of monitoring and mitigation is discussed. There is an urgent need for more data on effects of strong seismic pulses o n seals.