Little is known about the occurrence of cetaceans found in offshore waters in the Gulf of Alaska; however, whaling records and a few recent surveys have shown this area to be important habitat. The United States Navy maintains a maritime training area in the central Gulf of Alaska, east of Kodiak Island, and has requested additional information on marine mammal presence and use of this area. To determine the occurrence and distribution of marine mammals in and around the Navy training area, a line-transect visual and acoustic survey was conducted 10-20 April 2009 from the NOAA R/V Oscar Dyson. The primary survey area encompassed nearshore, shelf and offshore pelagic waters of the central Gulf of Alaska. Survey lines were designed to provide equal coverage of the nearshore and offshore habitat.
During this project, the visual survey covered a total of 760 kilometers (410 nautical miles) on-effort while transit and fog effort legs accounted for 553 km (298 nm). There were a total of 96 sightings (453 individuals) of 11 confirmed marine mammal species; these included fin, humpback, gray, and minke whales as well as killer whales, Dall’s and harbor porpoise, Pacific white-sided dolphins and Steller sea lions, harbor seals and sea otters. Additionally, there were 36 sightings (46 individuals) of unidentified large whales, dolphins and pinnipeds. Acoustically, operations were conducted 24-hours/day surveying a total of 3,519 km (1,900 nm) and recording 49 acoustic detections of sperm whales and killer whales. Photographs of nineteen individual killer whales and four fin whales were obtained on this cruise and compared to existing catalogs.
Density and abundance estimates were calculated for fin and humpback whales by stratum using several models. All results were fairly similar given the constraints of the sample sizes involved. Best estimates were obtained using additional sightings from a previous cruise on a similar vessel for calculating the sighting detection function (distances at which whales were sighted from the transect line). These yielded estimates of 594 (CV=0.29) and 889 (CV=0.57) fin whales for the inshore and offshore stratum, respectively, and 219 (CV=0.57) and 56 (CV=0.57) humpback whales in the inshore and offshore stratums, respectively. A small proportion of large whales were not identified to species but were most likely fin or humpback whales and estimates of these unidentified whales could be assigned to these species based on the proportion of fin and humpback whales identified in each stratum. This raised fin whale estimates of abundance to 666 and 938 (inshore and offshore stratums, respectively) and humpback whale estimates to 265 and 63 (inshore and offshore stratums, respectively).
Despite a number of logistical and time limitations, the survey achieved its primary objectives and provided new information on marine mammal occurrence and abundance in the region. Sightings were adequate to allow density and abundance estimates for fin and humpback whales. Identification photographs obtained on this cruise provide verification of seasonal presence of individual fin and killer whales in a study area that is rarely accessible.
Citation:
Rone, B.K., A.B. Douglas, P. Clapham, A. Martinez, L.J. Morse, A.N. Zerbini, and J Calambokidis. 2009. Final report for the April 2009 Gulf of Alaska line-transect survey (GOALS) in the Navy training exercise area. For Contract N00244-09-P-0960 from Naval Postgraduate School with funding from N45. October 2009.(See Rone et al. 2010 for final publication of this work).
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