Correcting for missed observations in a shore-based count of whales

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 operating simultaneously and the second on locations of whales detected acoustically 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, depending on distance of the whale offshore from the observation site, visibility number of observers, and estimation approach. Neither approach fully accounted for heterogeneity in detection probabilities. Errors in determining which detections belonged to the same whale could lead to bias. Other problems 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.