HARBOR PORPOISE (PHOCOENA PHOCOENA) ABUNDANCE IN ALASKA: BRISTOL BAY TO SOUTHEAST ALASKA, 1991‐1993

Abstract

Between 1991 and 1993, Alaska harbor porpoise (Phocoena phocoena) abundance was investigated during aerial surveys throughout much of the coastal and offshore waters from Bristol Bay in the eastern Bering Sea to Dixon Entrance in Southeast Alaska. Line‐transect methodology was used, and only those observations made during optimal conditions were analyzed. Survey data indicated densities of 4.48 groups/100 km2, or approximately 3,531 harbor porpoises (95% C. I. 2,206‐5,651) in Bristol Bay and 0.54 groups/100 km2, or 136 harbor porpoises (95% C. I. 11‐1,645) for Cook Inlet. Efforts off Kodiak Island resulted in densities of 1.85 groups/100 km2, or an abundance estimate of 740 (95% C. I. 259‐2,115). Surveys off the south side of the Alaska Peninsula found densities of 2.03 groups/100 km2 and an abundance estimate of 551 (95% C. I. 423‐719). Surveys of offshore waters from Prince William Sound to Dixon Entrance yielded densities of 4.02 groups/100 km2 and an abundance estimate of 3,982 (95% C. I. 2,567‐6,177). Combining all years and areas yielded an uncorrected density estimate of 3.82 porpoises per 100 km2, resulting in an abundance estimate of 8,940 porpoises (CV = 13.8%) with a 95% confidence interval of 6,746‐11,848. Using correction factors from other studies to adjust for animals missed by observers, the total number of Alaska harbor porpoises is probably three times this number.

Harbor Porpoise, Phocoena phocoena vomerina, in Cook Inlet, Alaska

“…26-830) was estimated from this effort (Agler et al 11 :121), with the caveat that surveys were designed to detect seabirds and sea otters within 100 m of the vessel and observers were also searching the air 100 m above the vessel. The issues raised by Dahlheim et al (2000) also apply regarding porpoise behavior, suggesting this estimate may be negatively biased as well. Porpoise were not seen during the winter aerial survey (Appendix Fig.…”

“…Data are presented in a chronological order and in some cases multiple datasets are presented within one map. Estimates of harbor porpoise abundance and sighting rates (Dahlheim et al, 2000;Speckman and Piatt, 2000;Hobbs and Waite, 2010;Agler et al 11 ) are also described.…”

“…Only one dedicated survey for harbor porpoise has been undertaken in Cook Inlet; an aerial, line-transect survey conducted over 2 days in August 1991 covering 1,873 km (Dahlheim et al, 2000). However, seabird and marine mammal surveys have occurred frequently in different seasons and different areas of Cook Inlet.…”

“…11-1,645; coeffi cient of variation (CV) = 63.2%). Dahlheim et al (2000) concluded that because counts were not corrected for animals missed on the trackline and all areas were not surveyed (in Cook Inlet this included the coastline and many bays), abundance estimates were likely biased downward. Other compromising factors, such as porpoise surfacing rates, the possibility of clumped distributions, and attraction to or avoidance of the survey platform, were also raised.…”

“…During 1-2 August 1991, 1,873 km along a single zigzag track line was fl own in Cook Inlet at an altitude of 500 ft (152.4 m) during surveys designed to enumerate harbor porpoise (Dahlheim et al, 2000) (Fig. 11, Table 1).…”