Hobbs and Waite: Abundance of Phocoena phocoena in three Alaska regions 
267 
Observations from the side observers and belly observer 
were combined and duplicates removed for density esti- 
mation so that the perception bias for the observer team, 
P t (Y t ,Y b ,Y . \) , (where 1, b, and r are the left side, belly, and 
right side observers, respectively) was the probability 
that at least one of them would perceive the group. The 
visual field of the side observers was treated as though 
their field of view ended at 90° from the horizontal on 
each side. These observers were in open communication 
and duplicates were resolved during the survey, so that 
each observer effectively watched half of the survey 
trackline. Thus, gdO) for any transect segment with 
constant environmental conditions was 
g(0) = l-[l-P(Y 6 )] 
p(r,)+p(y r ) 
2 
Variance was estimated by the delta method as 
where 
Da 
{Yl,Y b ,Y r 
d(3 
i zA 
2 
[{y b P b + yi Pi){\~Pi) + {ybPb + y r P r ){l-P r )\ 
with r B - the variance-covariance matrix for B esti- 
mated during the logistic regression; 
D b = the vector of partial derivatives of P t with 
respect to the coefficients (B). 
