Assessing ‘observer effects’ from an aircraft on behavior of three Delphinidae species (Grampus griseus, Delphinus delphis, and Orcinus orca)

Abstract

We systematically video-documented three species of Delphinidae: Risso’s dolphin (Grampus griseus) short-beaked common dolphin (Delphinus delphis) and killer whales (Orcinus orca) in the Southern California Bight (2009-2011) to assess whether the observation aircraft (fixed-wing Partenavia) affected selected behavioral variables. Focal observations were conducted to examine potential changes in group cohesion and heading reorientation rate, to the plane circling at four altitudes; 213m, 305m, 457m, and 610m while maintaining a radial distance >500 m. Paired t-tests were used to test the null hypothesis that mean maximum cohesion and mean reorientation of groups do not vary significantly based on plane altitude. For cohesion no significant effects were found for the eight G. griseus focal sessions (p = 0.447), one D. delphis (p = 0.602) and one O. orca (p = 0.197). For reorientation no significant effects were found for the eight G. griseus focal sessions (p = 0.591) and one O. orca (p = 0.936); the sample size was too small to calculate reorientation for D. delphis. Our results suggest that our small plane circling at radial distance >500 m and altitude 213 – 610 m did not cause measurable changes in cohesion and reorientation or other observable changes for the three species. We believe this is due to the aircraft remaining >500 m radial distance from the animals and at altitudes well outside the theoretical 26-degree sound transmission cone (“Snell’s Cone”) below the aircraft for the air-through-water interface.