Fishery Bulletin 96(4), 1 998 
proportion of time. Barlow (1988) reported that har- 
bor porpoise quickly avoid a closely approaching sur- 
vey vessel. Vessel avoidance by harbor porpoise may 
result in animals remaining undetected by observ- 
10.5 II 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 >16 
Temperature (°C) 
[ | Frequency of porpoise sightings 
□ Expected frequencies (boat survey temperatures) 
Figure 9 
Sea surface temperatures (°C) recorded at harbor porpoise 
sightings (n= 267) in relation to expected distribution of harbor 
porpoise if they were distributed randomly with temperature (as 
determined from temperatures at 427 locations along 73 random 
transect lines). An asterisk (*) designates a significant (P<0.05) 
difference, determined using chi-square goodness-of-fit analyses. 
ers or may affect estimates of perpendicular distance 
of harbor porpoise from the vessel. If the frequency 
of harbor porpoise sightings were greatest near the 
trackline and decreased with increasing perpendicu- 
lar distance during this study, it appeared that 
most harbor porpoise were detected before po- 
tentially significant vessel avoidance occurred. 
If porpoise did avoid the vessel, our abundance 
estimates would be underestimated. Ship avoid- 
ance was likely constant throughout the sur- 
vey period, however, and would not have af- 
fected results of habitat correlates. 
Density estimates of harbor porpoise (1.26 
porpoise/km 2 ) within the study area (237 km 2 ) 
were greater than densities reported by 
Calambokidis et al. 2 (0.42 porpoise/km 2 ) for 
waters off the San Juan Islands and part of the 
Strait of Georgia (2291 km 2 ) but were similar 
to density estimates of Flaherty and Stark 1 
(0.85 to 1.63 porpoise/km 2 ) for the north and 
west San Juan Islands (1005 km 2 ). Density es- 
timates reported by Calambokidis et al. 2 were 
based on an initial g( 0) equal to 0.324 (CV= 
0.171) multiplied by a correction factor of 3.1 
and yielding a g(0) of one. Green et al. 5 sur- 
veyed an extensive area within the 100-m 
isobath off the coast of Oregon and Washington 
and also reported a much lower density of har- 
bor porpoise (0.17 porpoise/km 2 ) than reported 
here. These differences probably result from 
Green et al. 5 and Calambokidis et al. 2 includ- 
ing regions of high and low harbor porpoise 
abundance in contrast to our focus on high 
density areas off the northern San Juan 
Islands. 
Prey or habitat requirements often limit 
distribution of cetaceans to regions that 
may vary daily, seasonally, or yearly, de- 
pending on an individual’s foraging, mat- 
ing, or behavioral requirements. During 
this study, surveys were conducted only 
within the summer months (June to Octo- 
ber) and thus may account for the rela- 
tively high density estimates of harbor 
porpoise within our study area. Flaherty 
and Stark 1 sighted harbor porpoise dur- 
ing all months of the year off the San Juan 
5 Green, G. A., J. J. Brueggeman, C. E. Bowlby, R. A. 
Grotefendt, M. L. Bonnell, and K. T. Balcomb 
III. 1992. Cetacean distribution and abundance 
off Oregon and Washington, 1989-1990. In J. J. 
Brueggeman (ed. ), Final report prepared by Ebasco 
Environmental and Ecological Consulting, Inc. for 
Minerals Management Service, Pacific OCS 
Region. Offshore Continental Shelf (OCS) Study 
MMS 91-0093, 100 p. 
Figure 10 
Mean number of harbor porpoise observed per minute during random 
boat surveys (June to October 1992) for all sections (A-E) combined. 
Vertical lines represent standard error. 
