Counts of northern elephant seals, Mirounga angustirostris, from large-format aerial photographs taken at rookeries in Southern California during the breeding season

Various techniques have been used to count northern elephant seals, Mirounga angustirostris, at rookeries in the United States and Mexico. Individuals are customarily counted by observers stationed in observation blinds or on walks atop cliff-tops, along the beach, or among the seals. Some rookeries are counted from skiffs nearshore. Large-format (228-mm) black-and-white or small-format (35-mm) color-transparency aerial photographs also have been used to count northern elephant seals. In addition, counts from aerial photographs have been combined with information on the phenology of haulout behavior to estimate elephant seal abundance when they are not censused at peak haulout periods. In 1985, scientists of the National Marine Fisheries Service began to count northern elephant seals from vertical, color-transparency photographs taken with a large-format camera. A 228-mm-format (cartographic) camera was used in 1985 and 1986, but an improved system was adopted in 1987 which used a 126-mm-format (military reconnaissance) camera adjusted for low altitude photography. This paper describes equipment and procedures used for counting northern elephant seals from photographs taken with these cameras. Counts of M. angustirostris are presented for San Miguel Island (1985-95), San Nicolas Island (1988-95), Santa Rosa Island (1990-95), and Santa Barbara Island (1993-95) off the coast of Southern California. The method used to obtain these counts was validated by comparing the precision of counts of northern elephant seals made by biologists on the ground with counts made from large-format aerial color-photographs. The counts of pups that were obtained from photographs taken at each island were then used to estimate the number of births for each year. These birth estimates were then compared with published estimates where other techniques were used in order to evaluate the results obtained by each technique.