Numbers of pups are estimated every other year using mark-resighting methods. Pups are marked in early August when they are about 1 month old. On St. George Island, estimates are obtained on all rookeries and on St. Paul Island, on a sample of about half the rookeries. Later, biologists visit the rookeries and count numbers of marked and unmarked animals in samples of groups of pups. The total number of animals on each rookery is estimated from the known number of marked animals and the proportion of marked animals in the samples.
Biologists also traverse the sites and count the number of dead pups.
These counts, together with the mark-resighting estimates, provide an estimate of the total number of pups born for each rookery. Estimates of number of northern fur seal pups born on St. George and St. Paul Islands, Alaska, 1970-98. Estimated numbers of northern fur seal pups born on St. Paul and St. George Islands in 1996 and 1998. These estimates were obtained using mark-resighting methods. The estimates do not include numbers from Sea Lion Rock, a small reef 200 m off St. Paul Island.