Ecological aspects of transient killer whales (Orcinus orca) as predators in southeastern Alaska

 

Wildlife Biology, Volume 6, No. 3:  308-322

 

Marilyn E. Dahlheim, National Marine Fisheries Service, Alaska Fisheries Science Center, National Marine Mammal Laboratory, 7600 Sand Point Way NE., Seattle, Washington 98115. USA.  E-mail:  marilyn.dahlheim@noaa.gov     

 

Paula A. White, Museum of Vertebrate Zoology, 3101 Valley Life Sciences Building, University of California, Berkeley, California 94720.  USA.

 

Abstract

     In this study we present empirical data on predator numbers, movements and area usage, and predation obtained from tracking transient killer whales (Orcinus orca) throughout the inland waters of southeastern Alaska, USA.  During 1991-2007, we documented 155 transient killer whales via photo-identification methodology within this large study area (27,808 km2).  Transient killer whales were distributed throughout southeastern Alaska and were present during all seasons, although not all individuals were seen each year.  Resighting data suggested that within southeastern Alaska, maternal groups may partition area usage of their environment.  By following whales for 1,467 km, we calculated a mean travel speed of 7.2 km/h with mean daily movements of 134 km ± 88 km/24 hours and ranging within 59-240 km/24 hours.  Photographic matches demonstrated that most (86%) of the transient killer whales identified in southeastern Alaska also utilized British Columbia and Washington State waters.  In contrast, photographic matches between whales in southeastern Alaska and whales seen off of California, USA, were rare, suggesting that different transient killer whale stocks occupy these two regions.  Transient killer whales preyed upon Dall’s porpoise (Phocoenoides dalli), Pacific white-sided dolphins (Lagenoryhncus obliquidens), harbor porpoise (Phocoena phocoena), minke whales (Balaenoptera acutorostrata), Steller sea lions (Eumetopias jubatus), harbor seals (Phoca vitulina), and seabirds.  Potential prey species available, but not targeted, included humpback whales (Megaptera novaeangliae), elephant seals (Mirounga angustirostris), and sea otters (Enhydra lutris).  Prey-handling techniques varied depending upon the prey being targeted with no evidence of prey specialization.  During 114 encounters totaling 332.5 hours of direct observations of transient killer whales, we documented 36 predation events for a calculated kill rate of 0.62 prey items/24-hour period/whale.  The data we present in this article provide a foundation of transient killer whale ecology aimed at improving our ability to understand the impact of transient killer whale predation on southeastern Alaska prey populations.