Raum-Suryan and Harvey: Distribution and abundance of and habitat use by Phocoena phocoena 
817 
sels underestimated true group size of harbor 
porpoise, missing up to 60% or more of animals 
(Calambokidis et al. 4 ). However, these results 
were based on a very small sample size and the 
survey vessel traveled at twice the speed of our 
vessel. In our study, mean group size (1.91 por- 
poise) was only 5% different from that detected 
from concurrent shore-based surveys (2.01 por- 
poise; Raum-Suryan, 1995). We are, therefore, 
confident that any biases in group size estimates 
are small. 
Accurately measuring the locations of marine 
mammals from vessels can be affected by the 
height of observers above water (Polacheck and 
Smith, 1989) and the use of reticle and compass 
binoculars (Smith, 1982; Barlow and Lee, 1994). 
From a low height above the water, angles are 
greatly affected by small deviations in reticle 
estimates. As radial distances to harbor porpoise 
decrease, however, errors in reticle estimates 
(sighting angles) have progressively less effect 
on distance calculations. The majority (78%) of 
our radial and perpendicular 
sighting distances were less than 
350 m. At 350 m an error of ±0.1 
reticle was equal to 50 m (the 
range of our data groupings which 
best fitted the model). Therefore, 
although the platform height of 
our survey vessel was low (2.68 m), 
we were able to obtain accurate 
sighting data by conducting sur- 
veys only during optimal sighting 
conditions (Beaufort <1), and be- 
cause there was both a lack of 
ocean swell and the majority of 
sightings were less than 350 m 
distant. 
Harbor porpoise are small, in- 
conspicuous animals that avoid 
boats (Amudin and Amudin, 1974; 
Gaskin, 1977; Prescott and Fiorelli, 
1980; Barlow, 1988). Detection of 
harbor porpoise before they be- 
come aware of the survey vessel is 
often difficult without prior knowl- 
edge of their locations. Polacheck 
and Thorpe (1990) observed har- 
bor porpoise swimming away from 
their survey vessel a significant 
90- 
80- 
70- 
60- 
50- 
« 40- 
30- 
20 - 
10 - 
n= 120 
n = 96 
n= 120 
= 136 
Section A Section B Section C Section D Section E 
Figure 7 
Mean percentage slope of seafloor for each section (A- E) de- 
termined from random boat surveys. Vertical lines represent 
standard error and “n” represents the number of random slope 
locations plotted within each section (total=584). 
8 10 12 14 16 18 20 22 24 26 >26 
12 14 16 
Percentage Slope 
Observed porpoise □ Expected porpoise 
Figure 8 
Harbor porpoise sightings (n= 275) from random boat surveys (June— October 1992) 
in relation to expected distribution if porpoise were distributed randomly with slope 
(as determined from slopes at 584 random locations). An asterisk (*) designates a 
significant (P<0.05) difference, determined with chi-square goodness-of-fit analyses. 
4 Calambokidis, J., S. R. Melin, and D. J. Rugh. 1991. Land- 
based calibrations of harbor porpoise sightings from a vessel 
along the northern Washington coast. In H. Kajimura ( ed. ), 
Harbor porpoise interactions with Makah Salmon set net fish- 
ery in coastal Washington waters, 1988-89. National Marine 
Mammal Laboratory Processed Report. National Marine Mam- 
mal Laboratory, Natl. Mar. Fish. Serv., NOAA, 7600 Sand Point 
Way NE, Seattle, WA 98115-0070. 
