Counting Steller sea lion pups in Alaska: An evaluation of medium-format, color aerial photography

Estimates of Steller sea lion (Eumetopias jubatus) pup production are valua ble for estimating population trend and size. Currently in Alaska, pups are counted by visiting rookeries, driving older animals into the water, then walking through the rookeries and counting the pups, a highly disruptive pr ocedure. At smaller rookeries, with good vantage points, pups are occasiona lly counted from the periphery of rookeries without disturbing the sea lion s. We evaluated counts made from medium-format, color, aerial photographs a s an alternative to drive counts and peripheral counts. Neither the periphe ral counts nor the aerial photographic counts disturbed animals on the rook eries. There were strong 1:1 linear relationships between photographic coun ts and drive counts (r(2) = 0.966, P < 0.001) and between photographic coun ts and peripheral counts (r(2) = 0.999, P < 0.001). Precision was similar f or all three methods of counting. We suggest that medium-format, color, aer ial photography is appropriate for routine surveys of Steller sea lion pups in Alaska because it is not disruptive to the hauled-out sea lions and pro vides comparable estimates with similar precision to drive and peripheral c ounts. Large areas can be rapidly surveyed during periods of good weather w ith a minimum of manpower.