260 
Fishery Bulletin 108(3) 
Table 1 
Estimated g(0) (probability of detecting an animal at the surface on the trackline (perception bias) and effective strip width 
(ESW) in km for harbor porpoise in Alaska determined from surveys from 1997 through 1999. Data were obtained from indi- 
vidual observers and teams of observers, all of whom reported the same visibility code. The product of these two values, gtO) and 
ESW, is a measure of the relative effectiveness of the observer team under different conditions. Single observer=single observer 
at either the right, left, or belly window of aircraft. Team of observers=a team of observers at the right, left, and belly window 
of the aircraft. 
Single observer Team of observers 
1997 1998-99 1997 1998-99 
Visibility 
Value 
SE 
Value 
SE 
Value 
SE 
Value 
SE 
1 (excellent) 
g<0) 
0.66 
0.08 
0.73 
0.06 
0.89 
0.01 
0.93 
0.01 
ESW 
0.252 
0.022 
0.139 
0.008 
0.252 
0.022 
0.118 
0.007 
g(0)ESW 
0.166 
0.009 
0.101 
0.004 
0.224 
0.002 
0.11 
0.001 
2 (good) 
g(0) 
0.55 
0.06 
0.62 
0.04 
0.79 
0.02 
0.85 
0.01 
ESW 
0.2 
0.017 
0.139 
0.008 
0.2 
0.017 
0.118 
0.007 
g(0)ESW 
0.11 
0.004 
0.086 
0.002 
0.158 
0.003 
0.1 
0.001 
3 (fair) 
g(0) 
0.42 
0.07 
0.49 
0.06 
0.66 
0.04 
0.74 
0.02 
ESW 
0.153 
0.013 
0.139 
0.008 
0.153 
0.013 
0.118 
0.007 
g(0)ESW 
0.064 
0.002 
0.068 
0.002 
0.101 
0.003 
0.087 
0.001 
4 (poor) 
£(0) 
0.31 
0.1 
0.37 
0.09 
0.52 
0.11 
0.61 
0.08 
ESW 
0.116 
0.01 
0.139 
0.008 
0.116 
0.01 
0.118 
0.007 
g(0)ESW 
0.036 
0.001 
0.051 
0.002 
0.06 
0.003 
0.072 
0.004 
5 (unacceptable) 
£(0) 
0.21 
0.11 
0.26 
0.12 
0.38 
0.19 
0.46 
0.16 
approximately 30% decrease in effective effort with 
each step in visibility and that survey effort during poor 
conditions had less than one quarter of the effective- 
ness of effort during the best conditions (Fig. 7). In the 
1998-99 surveys (Fig. 8), the ESW was narrower overall 
and slightly broader when the belly observer was not 
present. Although this seems counterintuitive, it is the 
result of a peak that occurred near the trackline when 
the belly sightings were included and which made the 
distribution away from the trackline relatively lower and 
resulted in the narrower ESW (Table 1, Fig. 8). When 
ESW andg(O) were multiplied together, the added value 
of the belly observer was 10% under the best conditions 
and nearly 50% under poor conditions. 
Density and abundance of harbor porpoise 
Abundance estimates of harbor porpoise increased from 
east to west as did estimates of average density by stock 
(0.10, 0.19, and 0.44 porpoise/km-, respectively). Aver- 
age observed harbor porpoise densities (uncorrected for 
availability or perception biases) for the SEA, the GOA, 
and the BS stocks were 0.033 groups/km 2 (CV=17.2%), 
0.062 (CV=11.9%), and 0.153 (CV=13.2%), respectively. 
Approximately 5% of the study areas, consisting primar- 
ily of inlets and channels, were unsurveyed. Density 
estimates for these unsurveyed areas were extrapolated 
from similar surveyed areas in the same general region 
(Table 3). The correction factor of 2.96 (CV=0.180) (Laake 
et al., 1997) was applied to each abundance estimate to 
account for availability bias. The full corrections for vis- 
ibility bias (correction for perception bias x correction for 
availability bias) were 4.62 (SEA, CV=21%), 4.06 (GOA, 
CV=19%), and 3.96 (BS, CV=19%). 
For the Cook Inlet survey, the effective strip width 
(0.280 km, CV=0.281) was based on 44 sightings from 
the 1993 tol999 surveys. Truncation of the sighting 
strip by discarding sightings less than 0.1 km from the 
trackline or greater than 0.6 km from the trackline on 
each side of the plane was necessary to obtain a good fit 
of the detection function. The best fit for the detection 
function for the Cook Inlet data, based on AIC, was a 
hazard-rate curve with a cosine correction (Fig. 9). The 
1998 beluga whale survey in Cook Inlet resulted in 
eight harbor porpoise sightings along 1355 km of track- 
line. No data were available to estimate the perception 
bias for this survey and its format was sufficiently dif- 
ferent from the harbor porpoise survey with the result 
that it was uncertain whether the perception bias cor- 
rection would be approximately correct. Consequently, 
only the correction for availability (2.96, CV= 0.180) was 
applied. This results in a rather conservative estimate 
with a known negative bias which we feel is preferable 
to one with an unknown bias. 
The abundance estimate for the SEA stock of harbor 
porpoise was 11,146 animals (CV=24.2%; N min =9116, 
