NOTE Lowry et al Counts of Mirounga angustirostns from aerial photographs 



183 



cause of pup mortality [Le Boeuf and Reiter, 1991; 

 Stewart and Yochem, 1991]); 3) differences in esti- 

 mates of precensus pup mortality; and 4) the infe- 

 rior resolution of small-format ( 35-mm ) photographs 

 ( Glegg and Scherz, 1975 ) when they are used in aerial 

 surveys to estimate total number of births. While 

 differences exist between estimates of the number 

 of births, at this time there is no significant differ- 

 ence in the long-term trend between estimates de- 

 rived from ground counts or from counts made from 

 large-format aerial color photographs. 



A problem with ground surveys is that counts of 

 pups are repeated when they differ by 5% to 10% 

 (Stewart, 1989; Stewart et al., 1994). Large-format 

 aerial color photography makes it possible to obtain 

 accurate and precise counts of northern elephant 

 seals without additional survey effort. In addition, 

 the aircraft did not appear to disturb elephant seals 

 and other sympatric species of pinnipeds (no animals 

 abandoned the haulout area). 



The topography at San Miguel Island, San Nicolas 

 Island, and the western end of Santa Rosa Island is 

 ideal for conducting vertical aerial photographic cen- 

 suses of northern elephant seals because the coast- 

 lines of these islands lack tall vertical cliffs. High 

 vertical cliffs that are adjacent to beaches obscure 

 elephant seals by their overhangs and by the shad- 

 ows that they create. The topography at Santa Bar- 

 bara Island is not ideal for aerial photographic 

 censusing because cliffs that range up to 122 meters 

 in height obscure elephant seals at the haulouts be- 

 low them. Counts obtained from aerial photographs 

 at Santa Barbara Island may result in an underesti- 

 mate of the number of pups present (Table 5). 



Counts of northern elephant seals obtained from 

 ground and small-vessel surveys require calibration 

 against counts obtained from vertical aerial color 

 photographs taken with a large-format, IMC- 

 equipped camera. These calibrations are needed for 

 each rookery to evaluate the effects of rookery size 



