An aerial survey method to estimate sea otter abundance

Sea otters (Enhydra lutris) occur in shallow coastal habitats and can be hi ghly visible on the sea surface. They generally rest in groups and their de tection depends on factors that include sea conditions, viewing platform, o bserver technique and skill, distance, habitat and group size. While visibl e on the surface, they are difficult to see while diving and mag dive in re sponse to an approaching sun ey platform. We developed and tested an aerial survey method that uses intensive searches within portions of strip transe cts to adjust for availability and sightability biases. Correction factors are estimated independently for each survey and observer. In tests of our m ethod using shore-based observers, we estimated detection probabilities of 0.52-0.72 in standard strip-transects and 0.96 in intensive searches. We us ed the survey method in Prince William Sound, Alaska to estimate a sea otte r population size of 9,092 (SE = 1422). The new method represents an improv ement over various aspects of previous methods, but additional development and testing will be required prior to its broad application.