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

Abstract

Killer whale (Orcinus orca Linnaeus, 1758) abundance in the North PaciWc 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 con- ducted 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 abun- dance of diVerent killer whale ecotypes, which were distinguished based upon morphological and genetic data. Abundance was calculated separately for two data sets that diVered 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 Wrst sighting, and post-encounter group size (PEGS) corresponded to estimates made after closely approach- ing sighted groups. 'Resident'-type (Wsh-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 'oVshore'-type killer whales were recorded during the surveys, one northeast of Unalaska Island and the other south of Kodiak Island. These are the Wrst 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.