Little is known about the present-day 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 U.S. 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 describe the occurrence and distribution of marine mammals in and around the U.S. Navy training area, a line transect visual and acoustic survey was conducted 10-20 April 2009 from the NOAA ship Oscar Dyson. The primary survey area encompassed nearshore 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 km (410 nautical miles, nmi) on-effort (visible horizon, Beaufort sea state 5 or less, and survey speed of 10 knots through the water) while transit (visible horizon, Beaufort sea state 5 or less, and survey speed of 12 knots) and fog effort (no horizon, Beaufort sea state 5 or less) legs accounted for 553 km (298 nmi). There were a total of 96 sightings (453 individuals) of 11 confirmed marine mammal species; these included fin (Balaenoptera physalus), humpback (Megaptera novaeangliae), gray (Eschrichtius robustus), minke (B. acutorostrata) whales, and killer whales (Orcinus orca), Dall’s (Phocoenoides dalli) and harbor (Phocoena phocoena) porpoise, Pacific white-sided dolphins (Lagenorhynchus obliquidens), Steller sea lions (Eumetopias jubatus), harbor seals (Phoca vitulina), and sea otters (Enhydra lutris). Additionally, there were 36 sightings (46 individuals) of unidentified large whales, dolphins, and pinnipeds. Passive acoustic operations were conducted 24 hours/day surveying a total of 3,519 km (1,900 nmi) and recorded 49 acoustic detections of sperm whales (Physeter macrocephalus) and killer whales. Photographs of 19 individual killer whales and 4 fin whales were obtained on this cruise and compared to existing photo-identification catalogs.
Density and abundance estimates were calculated for fin and humpback whales by stratum using line transect methods with and without covariates in detection probability models. Additional sightings from a previous cruise on a comparable vessel were used for improving estimation of detection probability. All results were fairly similar given the constraints of the sample sizes involved. Estimates of abundance in the inshore and offshore strata were 594 (CV = 0.29) and 889 (CV = 0.57) for fin whales, and 219 (CV = 0.57) and 56 (CV = 0.57) for humpback whales, respectively. A small proportion of large whales were not identified to species but were probable fin or humpback whales based on observations, and estimates of these unidentified whales were 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 (CV = 0.3) and 938 (CV = 0.57) and humpback whale estimates to 265 (CV = 0.48) and 63 (CV = 0.51, inshore and offshore strata, respectively).
Despite a number of logistical and time limitations, the survey provided new information on the occurrence and abundance of marine mammals in the region. Sighting sample sizes were adequate to provide density and abundance estimates for fin and humpback whales. Identification photographs obtained on this cruise also verify the seasonal presence of individual fin and killer whales in a study area that is rarely surveyed.
Citation:
Rone, B.K., A.B. Douglas, A.N. Zerbini, L.J. Morse, A. Martinez, P.J. Clapham, and J. Calambokidis. 2010. Results from the April 2009 Gulf of Alaska line-transect survey (GOALS) in the Navy training exercise area. NOAA Technical Memorandum NMFS-AFSC-209. 39 pp.
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