Hobbs and Waite: Abundance of Phocoena phocoena in three Alaska regions 
259 
Figure 5 
Relative probability of detection of harbor porpoise ( Phocoena phocoena) by distance 
from the survey trackline (km) determined with a half-normal model for the 1997 
sighting data of harbor porpoise in the Southeast Alaska stock region. The histogram 
shows the distribution of sightings in 0.05-km groups. 
first to the second and third years, and 1997 was the 
first year of a belly observer for the survey leaders. The 
belly observer had a significantly higher probability of 
sighting an available group than the side observers for 
any particular sighting, and the belly observers routinely 
reported better visibility than did the side observers. 
This difference in perception bias was accounted for by 
the difference in reported visibility. The logistic regres- 
sion coefficients were as follows: constant=1.187 ±0.542, 
=2.19); for year=0 for 1997 and 1 for 1998, 1999, coef- 
ficients. 296 ±0.290, (£=1.02); for visibility=l (excellent), 
2 (good), 3 (fair), 4 (poor), 5 (unacceptable) as a continu- 
ous variable, coefficient=— 0.502 ±0.217, (£=-2.31). The 
model for probability of sighting of an available group 
for a single observer is then 
1 . 187 + 0 . 296 y ear-0. 502visibility 
P(sighting\ year, visibility) = - - el , 187+0 . 296ytor _o.so 2l ,fe l -6ffi» • 
Heterogeneity in probability of sighting a harbor por- 
poise resulted in a decrease of a difference of roughly 
0.11 for each reduction in visibility and an increase 
by roughly 0.06 from 1997 to 1998-99 (Table 1). The 
average observed g( 0) values (perception bias only) for 
the SEA stock, the GOA stock, and BS stock of harbor 
porpoise were 0.641 ±0.069, 0.729 ±0.048, and 0.748 
±0.046, respectively, and yielded average perception bias 
correction factors of 1.560 (CVS. 108), 1.372 (CVS. 066), 
and 1.337 (CVS. 062), respectively (Table 2). 
Estimated strip width for observations 
Variation in effective strip width (ESW) occurred for 
the configuration of observers and visibility as reported 
by the observers. Few sightings occurred beyond 400 m 
and therefore this distance was chosen as the trunca- 
tion point for distance from the trackline and sightings 
beyond this distance were not included in the analysis. 
Effort was separated into effort with and without a belly 
observer. ESW without a belly observer was the ESW on 
one side of the plane covered by a side observer. ESW 
with a belly observer represented the effort of one side 
observer and half of the belly observer because the belly 
observer’s field of view was divided by the trackline 
(note that duplicate sightings were removed such that 
where sightings were reported by both the side and belly 
observer, only the side sighting record was used). The 
ESW for 1997 was significantly different from that of 
1998 and 1999; therefore they were treated separately 
(Table 1). In the 1997 data, significant variation in ESW 
was related to visibility level and in the 1998 and 1999 
data, to the presence or absence of the belly observer. 
The best fit for the detection function was a half-normal 
curve for the 1997 data set and a half-normal curve with 
a one term cosine correction for the 1998—99 data set 
(Figs. 5 and 6, Table 2). The ESW of the survey team 
in 1997 decreased by roughly 20% per step change in 
visibility. When this decrease in ESW was combined 
with approximately a 12% decrease in g(0) with each 
step in visibility, the product (Table 1) indicated an 
