Bowhead whales (Balaena mysticetus) are counted near Point Barrow, Alaska,
during their spring migration. Missed whales have been accounted for in two
different ways, the first based on data from two observation sites operati
ng simultaneously and the second on locations of whales detected acoustical
ly as well as visually. Estimates of the probability that observers at the
primary observation site missed a whale ranged from 0.3 to nearly 1, depend
ing on distance of the whale offshore from the observation site, visibility
number of observers, and estimation approach. Neither approach fully accou
nted for heterogeneity in detection probabilities. Errors in determining wh
ich detections belonged to the same whale could lead to bias. Other problem
s for combined acoustic and visual surveys include the failure of whales to
vocalize in some situations and negative correlation between acoustic and
visual detections that could cause positive bias in estimated probabilities
of missing whales and estimated abundance.