A Double-Survey Estimate for Sighting Probability of Sea Otters in California

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Abstract
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 sera 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. Estimates 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 probabilites of sighting were: resting = 0.990, foraging = 0.769, and other = 0.885. All animals missed by 1 observer team were in groups of .ltoreq.2 otters, wheveras 52% of the animals sighted during the study were in groups .ltoreq.3. Observer teams may have varied slightly in sighting ability, but this variation did not substantially affect population estimats.

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