DETECTION OF SEA OTTERS IN BOAT-BASED SURVEYS OF PRINCE-WILLIAM-SOUND, ALASKA

Boat-based surveys have been commonly used to monitor sea otter popula tions, but there has been little quantitative work to evaluate detecti on biases that may affect these surveys. We used ground-based observer s to investigate sea otter detection probabilities in a boat-based sur vey of Prince William Sound, Alaska. We estimated that 30% of the otte rs present on surveyed transects were not detected by boat crews. Appr oximately half (53%) of the undetected otters were missed because the otters left the transects, apparently in response to the approaching b oat. Unbiased estimates of detection probabilities will be required fo r obtaining unbiased population estimates from boat-based surveys of s ea otters. Therefore, boat-based surveys should include methods to est imate sea otter detection probabilities under the conditions specific to each survey. Unbiased estimation of detection probabilities with gr ound-based observers requires either that the ground crews detect all of the otters in observed subunits, or that there are no errors in det ermining which crews saw each detected otter. Ground-based observer me thods may be appropriate in areas where nearly all of the sea otter ha bitat is potentially visible from ground-based vantage points.