Raum-Suryan and Harvey: Distribution and abundance of and habitat use by Phocoena phocoena 
815 
Fixed boat surveys 
Fifty-six sightings of 92 harbor porpoise were 
recorded during 33 surveys (port or starboard) 
in 1991, and 69 sightings of 118 harbor por- 
poise during 24 surveys (both sides of vessel 
surveyed) in 1992 (Fig. 2). Harbor porpoise 
were sighted during 79% of surveys in 1991 
and 75% in 1992. Mean group size was 1.6 
harbor porpoise (SE=0.09, n=56) in 1991 and 
1.7 harbor porpoise (SE=0.127, n=69) in 1992. 
Distribution of harbor porpoise was patchy but 
similar between 1991 and 1992 (Fig. 2). 
The greatest number of harbor porpoise 
sightings recorded per survey were along 
transects 1, 2, and 5 in 1991 and transects 1 
and 5 in 1992 (Table 2; Fig. 2). The least num- 
ber of sightings were recorded for transects 3 
and 6 in 1991 and 1992 (Table 2; Fig. 2). Mean 
number of harbor porpoise sightings per sur- 
vey was not significantly different (P>0.05) 
between 1991 and 1992 for any of the fixed 
transects (Table 2). Sample sizes for all 
transects were low because of the limited sur- 
vey period (July to August). Given our low 
sample size (Cohen, 1988, requires a sample 
size of eight or more), we were unable to de- 
termine power. If eight samples of each fixed 
transect line had been taken, power to detect 
a difference in density between 1991 and 1992 
would still have been low (power<27% for all 
fixed transects except transect three which 
had<6% power). 
Section A Section B Section C Section D Section E 
n = 14 n = 11 n = 15 «=I5 n = 12 
b) 180 
Section A Section B Section C Section D Section E 
Figure 5 
Mean number of harbor porpoise (A) and mean water depth (B) of 
each section (A-E) determined during random boat surveys ( June- 
October 1992). Vertical lines represent standard error and “n” rep- 
resents number of transects completed in each section (A; total=73) 
and random depth locations plotted within each section (B); to- 
tal=584). 
Discussion 
Population and density estimates of harbor 
porpoise were based on several assumptions of line 
transect theory. Relevant assumptions included the 
following: 1) study area was sampled randomly 
(transect lines placed randomly with respect to the 
distribution of objects) or animals were randomly 
distributed; 2) all animals on the trackline were de- 
tected; 3) group size was estimated without error; 4) 
locations were measured accurately for each indi- 
vidual or group; and 5) animals did not move in re- 
sponse to the survey vessel or were detected before 
they moved (Burnham et al., 1980). 
The first assumption of line transect theory was 
met by employing a stratified random sampling de- 
sign within the study area. This design was chosen 
so that the 8-km transects would adequately cover 
the entire study area. By using fixed length straight 
transects, however, certain areas of sections B and 
C were not adequately sampled. The habitat features 
of these areas were similar to the rest of the study 
area, and portions of section C not sampled during 
random surveys were sampled during fixed transect 
surveys 5 and 6. A study design that incorporated 
shorter transects (4 km) would allow more complete 
coverage of all areas within strata. If this study were 
replicated, we recommend incorporating 4-km 
transects to cover the areas that we missed. We do 
not believe, however, that our study design affected 
the results of the habitat correlates. By randomly 
surveying within a defined region off the northern 
San Juan Islands, we adequately sampled oceano- 
graphic features of interest (depth, seafloor slope, 
surface temperature, tides). 
The assumption that all animals are detected on 
the trackline is often violated during marine mam- 
mal surveys. Animals with long durations of submer- 
