A double survey estimate for sighting probability of sea otters in California

We developed an estimator for the probability of sighting sea otters (Enhydra lutris) by shore- based counters, based on simultaneous double-surveys. We then estimated probability of sighting sea otters in California and evaluated the estimator's principal assumptions. The overall probability of sighting sea otters on 5 replicated double-surveys at each of 6 study areas was 0.945. Estimated probability of sighting did not vary (P > 0.05) among study areas or over time and was not correlated (P > 0.05) with distance of otters from observers. Probability of sighting was affected (P < 0.05) by sea otter activity and group size. Activity-specific probabilities of sighting were: resting = 0.990, foraging = 0.769, and other = 0.885. All animals missed by 1 observer team were in groups of ≤2 otters, whereas 52% of the animals sighted during the study were in groups ≥3. Observer teams may have varied slightly in sighting ability, but this variation did not substantially affect population estimates.