Caroline Weir. Overt Responses of Humpback Whales (Megaptera novaeangliae), Sperm Whales (Physeter macrocephalus), and Atlantic Spotted Dolphins (Stellena frontalis) to Seismic Exploration off Angola. Aquatic Mammals 2008, 34 (1), 71-83.

During ten months of seismic surveys off the Angola coast, 2769 hours of marine mammal observations were made from a survey vessel, seeking to determine whether marine mammals avoided the airgun noise. This study did not examine subtler responses, such as dive patterns or call rates, but simply tracked sighting rates and distances. The total number of marine mammal sightings was rather small, given the long timeframe (66 humpbacks, 124 sperm whales, 17 dolphins); the author does not offer any hints as to whether populations are simply low in that area, or whether observations were limited for any other reason (weather, single observer, high seas, etc.). Airguns were active roughly half the time, providing a balanced set of data to look at. The mean distance at which all species were seen was greater when airguns were active than when they were silent, though only the dolphins showed a statistically significant difference. The closest approach of humpbacks averaged 3000m with guns off and 2700m with guns on, with sperm whale results virtually identical; dolphins, by contrast, came much closer during guns-off, 209m, than when guns were on, 1080m. Perhaps unexpectedly, more whales were seen at virtually each distance charted (from 1km to 6km) when the airguns were on–while this could indicate a lack of avoidance behavior, it is also possible that it was caused by the whales staying at or near the surface while airguns were active, a “vertical avoidance” of higher sound levels at depth (near the surface, the airgun noise is reduced by destructive interference as sound bounces off the surface). Counter to expectations that the large whales would be more sensitive to the predominantly low-frequency airgun noise, the dolphins were much more dramatically affected. All nine times that dolphins approached the ship occurred only in airguns-off periods, and on one occasion, an approaching group of dolphins clearly veered away as the airguns ramped up. While the dolphins clearly avoided an area near the ship, there was no evidence that the seismic survey activity displaced any species from the region; sperm whale sightings increased over the course of the survey.