258 
Fishery Bulletin 108(3) 
164°W 160°W 156°W 
Completed survey transects and sightings (circles) of harbor porpoise ( Phocoena phocoena) during 
the 1999 aerial survey in the Bering Sea stock region. Transect and sightings on the south side 
of the Alaska Peninsula are shown in Figure 3 as part of the Gulf of Alaska survey. 
for the inexperienced observers in the belly, we did not 
have the estimate of the density for the other species 
necessary to complete this calculation. 
Group size was typically underestimated by the belly 
observer in comparison to the side observers. In sight- 
ings by both the belly and side observers, separate 
corrections were calculated for group sizes reported as 
one individual and groups reported as two individuals 
by the belly observer. Of 30 groups reported as one 
harbor porpoise by the belly observer, 25 were reported 
as a group size of one by the side observer and 5 were 
reported as a group size of 2, yielding a multiplicative 
correction of 1.167 (CV=0.059) groups of size one seen 
only by the belly observer. Likewise, of 12 groups re- 
ported as two harbor porpoise by the belly observer, 11 
were reported as a group size of 2 by the side observer 
and one was reported as a group size of 3, yielding a 
multiplicative correction of 1.042 (CV=0.080) for groups 
of 2 observed from the belly of the aircraft . The group- 
size estimate from the side observer was used when a 
group was reported by both the side and belly observers. 
The correction was only applied to group size when the 
group was seen only by the belly observer. A correc- 
tion for total animals was estimated as the sum of the 
group sizes with the belly-window-derived groups sizes 
corrected, divided by the sum of the group sizes with 
the belly-window-derived group sizes uncorrected. As a 
result, a multiplier of 1.018 (CV=0.006) was applied to 
the abundance estimates. It was necessary to apply a 
general correction rather than correct individual group 
sizes to avoid problems with the g(0) estimate and DIS- 
TANCE analysis arising from non-integer group sizes. 
Estimation of perception bias and g(0) 
Comparisons between sightings by the belly observer and 
the side observers indicated that each missed a small 
but significant fraction of the near-surface animals on 
the trackline. A total of 129 potential matches between 
experienced belly observers and independent side observ- 
ers within 50 m of the trackline were examined for 
perception bias and g(0) estimation. Although several 
of the potential covariates were significant by them- 
selves, only visibility as a continuous variable remained 
in the stepwise elimination. A significant difference in 
estimated strip width between survey year 1997 and 
the years 1998 and 1999 was identified in the distance 
analysis and, therefore, year was included as a covariate 
as well. Although year was not a significant coefficient, 
there was a significant turnover in personnel from the 
