Abstract

Boat surveys along randomly placed line transects were conducted from June to August 1991 and June to October 1992 to determine distribution and abundance of and habitat use by harbor porpoise (Phocoena phocoena) off the northern San Juan Islands, Washington. There were 301 sightings (average 4.4 sightings/h) of 526 harbor porpoise during 73 random boat surveys. with group sizes of 1 to 8 individuals (mean=1.87, SE=0.06, n=278). An estimated 299 harbor porpoise (1.26 porpoise/km2, SE=0.20) were distributed in an aggregated pattern within a 237 km2 area (10% of Washington Sound). indicating that a large proportion (30%) of harbor porpoise in Washington Sound occur in the northern San Juan Islands. Harbor porpoise were distributed over a depth range from 20.1 to 235.0 m (mean= 141.6 m, SE=2.43, n=275) and were observed more than expected (p<0.05) in depths greater than 125 m and over shallow slopes (<10%) and observed less than expected (p<0.05) in depths less than 75 m. Porpoise occurred at sea surface temperatures of 10.1° to 16.3°C and were sighted more frequently than expected (p<0.05) in water temperatures of 11° to 12°C. Boat surveys along fixed location transects indicated distribution was similar between 1991 and 1992. The occurrence of harbor porpoise in deep water, at cooler sea surface temperatures, over shallow sloping seafloor, and in tidally mixed regions (owing to currents and tide rips) within our study area may, collectively, affect prey distribution and associated harbor porpoise distribution.