Raum-Suryan and Harvey: Distribution and abundance of and habitat use by Phocoena phocoena 
809 
Random boat transects (n= 73) completed from June to October 1992 within five stratified 
sections (A, B, C, D, E), northern San Juan Islands, Washington. 
bor porpoise were observed more often during flood 
fide than ebb tide (Watson, 1976) and moved inshore 
during flood tides and offshore during ebb tides 
(Gaskin and Watson, 1985). 
Harbor porpoise in northern Puget Sound are vul- 
nerable to some of the same detrimental effects (dis- 
turbance, net entanglement, and pollution) that may 
have caused the disappearance of harbor porpoise in 
southern Puget Sound. It is important, therefore, to 
determine the abundance of harbor porpoise and 
identify habitat variables that may influence their 
distribution in northern Puget Sound. The main ob- 
jectives of this study were to determine 1) the spa- 
tial and temporal distribution, density, and abun- 
dance of harbor porpoise occurring off the northern San 
Juan Islands and 2) the relation of harbor porpoise to 
depth and percentage slope of the seafloor, sea surface 
temperature (SST), tidal state, and time of day. 
Methods 
Study area 
Washington Sound is located in the northwest cor- 
ner of Washington State (48°15’ to 48°5G’N and 
122°27 ! to 123°13’W), between the southern portion 
of Vancouver Island and the mainland, from Fidalgo 
Island to north of Vancouver, including the Ameri- 
can and Canadian islands of the San Juan Archi- 
pelago (Kozloff, 1973). Mean diurnal tide heights are 
between 1.3 and 2.9 m (NOAA, 1991). Northern 
Washington Sound (northern San Juan Islands) has 
numerous islands and reefs with deep channels, 
strong currents, and tide rips. The study area off the 
northern San Juan Islands (Fig. 1) was selected on 
the basis of preliminary boat surveys conducted in 
1991 to determine areas of harbor porpoise occur- 
rence. Additionally, information was obtained from 
local residents and The Whale Museum, Friday Har- 
bor, Washington. 
Random boat surveys 
Randomly located boat transects (n- 73; Fig. 1) were 
conducted from 27 June to 2 October 1992 within a 
study area composed of five strata (lettered A-E; Fig. 
1) to determine harbor porpoise distribution, den- 
sity, abundance, and habitat use. Eight-km transects 
were located within each approximately equal (42 to 
50 km 2 ) stratum by using random starting points and 
random compass bearings. Strata were originally 
chosen so that transects would adequately cover the 
entire study area. Because placement of straight 
8-km transects was constrained by the boundaries 
of strata and islands or reefs within strata, however, 
some regions of each strata were not adequately 
sampled. When sea conditions permitted, the five 
