Alternative approaches to estimation of harbor seal numbers: the role of spatial scale

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Complete counts of harbor seals (Phoca vitulina) hauled out during pupping season in Maine, USA, are expensive and logistically difficult. A complete aerial count of the haul-outs on the coast requires five to six days of flying. Given the short window of time around the peak of pupping, replicate counts are not feasible without considerable effort. Five previous coast-wide counts between 1981 and 2001 are evaluated to determine whether sample sites should be the same sites from year to year, or whether sites each year should be selected randomly. The information from previous counts is also used to define a sample unit and a sample design for the next aerial survey. Using previously observed haul-outs as sample units requires the assumption that no new haul-out sites will be colonized by harbor seals. In each of the previous surveys a substantial fraction of the sites where seals were observed were new haul-out sites. Also, some of the haul-out sites used during the previous survey were not used. Haul-out sites sometimes join adjacent haul-outs at very low tides, confounding the definition of a site for sampling. An alternative to using haul-out sites as sample units is to use 'bay-units', segments of the coast and ledges that can be counted in 90 min. or less. These units have been defined from previous surveys. They have the advantage that seals that are on new ledges will be included. With a bay-unit as a sample unit, a cluster sampling procedure is proposed for estimating the harbor seal population in the next aerial survey. Sample units would be selected with probability proportional to the number of ledges used by seals in previous surveys.