Overt Responses of Humpback Whales (<I>Megaptera novaeangliae</I>), Sperm Whales (<I>Physeter macrocephalus</I>), and Atlantic Spotted Dolphins (<I>Stenella frontalis</I>) to Seismic Exploration off Angola

Abstract

A total of 2,769 h of marine mammal observation was conducted from a seismic survey vessel off Angola between August 2004 and May 2005. A dual-source airgun array firing a total volume of either 5,085 in (Survey 1) or 3,147 in (Survey 2) in an alternate source activation sequence was active during 51% of the analysed effort. A total of 207 sightings of humpback whales (n = 66), sperm whales (n = 124), and Atlantic spotted dolphins (n = 17) was recorded. The encounter rate (sightings/h) of humpback and sperm whales did not differ significantly according to airgun operational status. The mean distance to humpback and sperm whale sightings was greater during full-array operations than during guns off, but this difference was not significant. Atlantic spotted dolphin encounters occurred at a significantly greater distance from the airgun array (p < 0.001) during full-array operations than during guns off. Positive-approach behaviour by Atlantic spotted dolphins (n = 9) occurred only during guns-off periods. There was no evidence for prolonged or large-scale displacement of each species from the region during the 10-mo survey duration. Sperm whale sightings showed a significant increase (p < 0.001) during the survey, while Atlantic spotted dolphin encounters occurred at similar rates. A decreased occurrence of humpback whale sightings (p < 0.001) corresponded with established seasonal migration out of the survey area. Contrary to expectation based on perceived sensitivity, Atlantic spotted dolphins exhibited the most marked overt response to airgun sound of the three cetacean species examined.