Marine Biology

, Volume 150, Issue 5, pp 1033–1045 | Cite as

Estimating abundance of killer whales in the nearshore waters of the Gulf of Alaska and Aleutian Islands using line-transect sampling

  • Alexandre N. Zerbini
  • Janice M. Waite
  • John W. Durban
  • Rick LeDuc
  • Marilyn E. Dahlheim
  • Paul R. Wade
Research Article

Abstract

Killer whale (Orcinus orca Linnaeus, 1758) abundance in the North Pacific is known only for a few populations for which extensive longitudinal data are available, with little quantitative data from more remote regions. Line-transect ship surveys were conducted in July and August of 2001–2003 in coastal waters of the western Gulf of Alaska and the Aleutian Islands. Conventional and Multiple Covariate Distance Sampling methods were used to estimate the abundance of different killer whale ecotypes, which were distinguished based upon morphological and genetic data. Abundance was calculated separately for two data sets that differed in the method by which killer whale group size data were obtained. Initial group size (IGS) data corresponded to estimates of group size at the time of first sighting, and post-encounter group size (PEGS) corresponded to estimates made after closely approaching sighted groups. ‘Resident’-type (fish-eating) killer whales were more abundant than the ‘transient’-type (mammal-eating). Abundance estimates of resident killer whales (991 [95% CI = 379–2,585] [IGS] and 1,587 [95% CI = 608–4,140] [PEGS]), were at least four times greater than those of the transient killer whales (200 [95% CI = 81–488] [IGS] and 251 [95% CI = 97–644] whales [PEGS]). The IGS estimate of abundance is preferred for resident killer whales because the estimate based on PEGS data may show an upward bias. The PEGS estimate of abundance is likely more accurate for transients. Residents were most abundant near Kodiak Island in the northern Gulf of Alaska, around Umnak and Unalaska Islands in the eastern Aleutians, and in Seguam Pass in the central Aleutians. This ecotype was not observed between 156 and 164°W, south of the Alaska Peninsula. In contrast, transient killer whale sightings were found at higher densities south of the Alaska Peninsula between the Shumagin Islands and the eastern Aleutians. Only two sightings of ‘offshore’-type killer whales were recorded during the surveys, one northeast of Unalaska Island and the other south of Kodiak Island. These are the first estimates of abundance of killer whale ecotypes in the Aleutian Islands and Alaska Peninsula area and provide a baseline for quantifying the role of these top predators in their ecosystem.

Notes

Acknowledgments

The authors thank Nancy Black, John Brandon, Dave Ellifrit, Jeff Jacobsen, Doug Kinzey, Kery Lodge, Lori Mazzuca, Kim Parsons, Bob Pitman, Michael Richlen, Susan Yin, and Ernesto Vazquez for their dedication and expertise in collecting data. The captains and crew of the Aleutian Mariner and the Coastal Pilot were very cooperative and contributed to the success of this study. Jeff Laake provided guidance and computer code for the data analysis. Kelly Robertson assisted with the DNA sequencing. Reviews by Steve Buckland, Kim Parsons, Glenn VanBlaricom and one anonymous reviewer were greatly appreciated. This study was part of the Ph.D. dissertation of A.N.Z., conducted under the supervision of Dr G. VanBlaricom at the School of Aquatic and Fishery Sciences (SAFS), University of Washington (UW). Funding was provided by the Brazilian Council for Scientific and Technological Development (CNPq, grant# 200.285/98-0), the National Marine Mammal Laboratory (NMML), and the Washington Cooperative Fish and Wildlife Research Unit, SAFS/UW.

Supplementary material

227_2006_347_MOESM1_ESM.pdf (55 kb)
Supplementary material

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • Alexandre N. Zerbini
    • 1
    • 2
  • Janice M. Waite
    • 2
  • John W. Durban
    • 2
  • Rick LeDuc
    • 3
  • Marilyn E. Dahlheim
    • 2
  • Paul R. Wade
    • 2
  1. 1.Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery SciencesUniversity of WashingtonSeattleUSA
  2. 2.National Marine Mammal LaboratoryNOAA Fisheries, Alaska Fisheries Science CenterSeattleUSA
  3. 3.Southwest Fisheries Science CenterLa JollaUSA

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