Correction of Visibility Bias in Aerial Surveys Where Animals Occur in Groups

Visibility is also affected when animals occur in groups, because larger groups have a higher probability of being observed than smaller groups. The result is an overestimation of mean group size and an underestimation of the mean number of groups per quadrat (Cook and Martin 1974). A general sighting function is proposed that relates the probability of sighting a group to its size. When ground counts are available on some of the quadrats, it is possible to estimate the parameters of the sighting function and obtain unbiased estimation of the mean group size, the density of groups, and the density of individuals. A proposed sighting function is applied to 3 aerial surveys of the sea otter (Enhydra lutris) in California, and also compared to a more restrictive sighting function suggested by Cook and Martin (1974).