250 
Fishery Bulletin 113(3) 
Table 4 
Sightings (AO of harbor porpoise ( Phocoena phocoena) and group encounter rates (ER; groups/km) during surveys conducted 
in summer in Southeast Alaska inland waters during the periods 1991-1993, 2006-2007, and 2010-2012. CV=coefficient of 
variation. 
Region 
Region name 
1991-1993 
2006-2007 
2010-2012 
N 
ER 
CV 
N 
ER 
CV 
N 
ER 
CV 
1 
Cross Sound, Icy Strait, and Glacier Bay 
177 
0.15 
0.12 
71 
0.14 
0.29 
160 
0.14 
0.13 
2 
Lynn Canal and Stephens Passage 
22 
0.03 
0.25 
7 
0.02 
0.39 
12 
0.02 
0.42 
3 
Frederick Sound 
25 
0.02 
0.28 
10 
0.02 
0.38 
11 
0.01 
0.51 
4 
Upper and Lower Chatham Strait 
16 
0.02 
0.29 
3 
0.01 
0.51 
4 
0.01 
0.00 
5 
Sumner Strait, Wrangell and Zarembo Island 
132 
0.16 
0.25 
39 
0.09 
0.28 
214 
0.19 
0.14 
6 
Clarence Strait to Ketchikan 
9 
0.02 
0.42 
- 
- 
- 
5 
0.01 
0.51 
Total 
381 
0.06 
0.12 
130 
0.05 
0.20 
412 
0.07 
0.17 
differ over the years, indicating that observer experi- 
ence was not an issue. 
Although experienced observers can readily discern 
the profile differences between a surfacing harbor por- 
poise and a slow rolling Dali’s porpoise (Phocoenoid.es 
dalli), accurate species identification can prove difficult 
for an inexperienced observer. Occasionally, a vertical 
spray of water, termed a “pop splash,” occurs as the 
porpoise breaks the water surface to breathe (Taylor 
and Dawson, 1984). This vertical spray of water gener- 
ated by a fast-moving harbor porpoise can, at times, 
be confused with the characteristic spray associated 
with a rooster-tailing Dali’s porpoise. Within the 2 ma- 
jor regions where harbor porpoise concentrated, Dali’s 
porpoise sightings were either rare or very limited in 
numbers (Dahlheim et ah, 2009). The percentage of un- 
identified porpoise varied across survey years from 0% 
to 16% of the total number of porpoise seen. Of particu- 
lar interest is that the percentages of unidentified por- 
poise were lowest (0-3%) during the mid-2000s, when 
the steepest abundance declines were noted. 
The experience level of observers varied during this 
multiyear study. However, on the basis of low varia- 
tion in group size, low overlap of harbor and Dali’s por- 
poise distribution, and the rate of unidentified porpoise 
sightings over the study period, the potential biases 
that result from observer variability are not a signifi- 
cant factor in the abundance estimates. 
Harbor porpoise are small, have no visible blow, and 
have a very low profile in the water. These features 
are well-known constraints in visual detection of this 
species (e.g., Hammond et al., 2002). Sighting cues can 
be very subtle, and observers can easily miss sighting 
an animal even in the best of conditions. The majority 
of the surveys in our study were conducted in Beau- 
fort sea states between 0 and 3 and that condition is 
reflected in the sighting data. Under these relatively 
good conditions, 99% of the sightings were collected. 
Nonetheless, models with a Beaufort category were se- 
lected in 2 out of the 3 periods, indicating that high- 
er sea states significantly reduced the sightability of 
porpoise. 
Over the 22-year period, 5 different vessels were 
used, but ship height did not influence detection of 
Table 5 
Average expected group size, E(S), for harbor porpoise (Phocoena phocoena) during surveys conducted in 
inland waters of Southeast Alaska during the summer season of the periods 1991-1993, 2006-2007, and 
2010-2012. CV=coefficient of variation 
1991- 
-1993 
2006-2007 
2010- 
-2012 
Region 
Region name 
E(S) 
CV 
E(S) 
CV 
E(S) 
CV 
1 
Cross Sound, Icy Strait, and Glacier Bay 
1.6 
0.06 
1.73 
0.06 
1.41 
0.06 
2 
Lynn Canal and Stephens Passage 
1.49 
0.10 
1.57 
0.17 
1.24 
0.07 
3 
Frederick Sound 
1.74 
0.10 
2.30 
0.19 
1.73 
0.14 
4 
Upper and Lower Chatham Strait 
1.39 
0.08 
1.33 
0.21 
1.50 
0.00 
5 
Sumner Strait, Wrangell, and Zarembo Island 
1.53 
0.04 
1.31 
0.05 
1.36 
0.03 
6 
Clarence Strait to Ketchikan 
1.61 
0.16 
0.00 
0.00 
1.00 
0.00 
Total 
1.56 
0.03 
1.59 
0.05 
1.37 
0.03 
