NOTE Lowry et al.: Counts of Mirounga angustirostris from aerial photographs 



177 



was mounted in a Partenavia P-68 aircraft ( 1987- 

 95). Both aircraft were flown at an altitude between 

 244 and 366 meters (typically 259 meters) and at a 

 ground speed of 90 to 110 knots. Kodak Aerochrome 

 MS Film 2448, a very fine-grain, medium-speed, 

 color-reversal film suitable for aerial photography, 

 was used. Photographs also were taken at an alti- 

 tude of 1,219 meters (at San Miguel Island in 1985) 

 and 1,372 meters (at San Nicolas Island and the 

 western end of Santa Rosa Island in 1989). These 

 high-altitude photographs were used 1 1 to determine 

 the geographic location of seals shown in low-alti- 

 tude photographs, 2) to prevent counting the same 

 seals twice that were visible along segments of the 

 coastline in photographs taken during two or more 

 photographic passes (double counting), and 3) to iden- 

 tify segments of coastline that were not photographed 

 during low altitude flights. 



Surveys 



Two aerial photographic surveys were conducted 

 during the northern elephant seal breeding season 

 (December through February) each year from 1985 

 to 1995, except 1987 and 1994, when only one sur- 

 vey was conducted. For each year, the first survey 

 occurred when the maximum number of adults was 

 expected to occupy rookery sites (between 18 and 30 

 January [Stewart, 1989] ). These surveys were sched- 

 uled for the later part of that period to maximize the 

 number of pups present, but winter storms or sched- 

 uling conflicts sometimes resulted in a later start- 

 ing date (Table 1). The second survey was made late 

 in the breeding-season, between 13 and 22 Febru- 

 ary, after all pups were born, but before weaned pups 

 had left the beaches (see Odell, 1974; Le Boeuf and 

 Bonnell, 1980; Stewart and Yochem, 1984; Stewart, 

 1989). 



Aerial photographic surveys were conducted off 

 southern California at San Miguel Island (34"02'N, 

 120°21'W) in 1985-95, San Nicolas Island (33°15'N, 

 119°30'W) in 1988-95, the western end of Santa Rosa 

 Island (34°00'N, 120°14'W) in 1990-95, and Santa 

 Barbara Island (33°29'N, 119°02'W) in 1993-95. A 

 peak breeding-season survey was not conducted at 

 San Nicolas Island in 1988 nor at Santa Rosa Island 

 in 1990. The southern shorelines of San Miguel Is- 

 land ( = 16 km) and San Nicolas Island ( = 18 km) were 

 photographed during all surveys. Portions of the 

 northern shorelines of San Miguel Island ( = 12 of =20 

 km) and San Nicolas Island (=4 of = 16 km) were in- 

 cluded in the surveys in 1988 and 1989, respectively. 

 At the western end of Santa Rosa Island, =4 km of 

 coastline were photographed, and at Santa Barbara 

 Island =0.75 km of 8 km of coastline were photo- 



graphed. A mosaic of overlapping photographs taken 

 from multiple photographic passes over the hauling 

 grounds at Point Bennett and Cardwell Point (located 

 on the western and eastern ends, respectively, of San 

 Miguel Island) was made to prevent double count- 

 ing seals that appeared in more than one photograph. 



Counts 



We counted six categories of elephant seals: 1) pups 

 that were alive or of unknown status (not decom- 

 posed, Fig. 1); 2) decomposed carcasses of pups; 3) 

 juveniles; 4) subadult and adult males; 5) adult fe- 

 males; and 6) seals of unknown age and sex category. 

 Color, shape, and behavior (i.e. their tendency to be 

 away from water) distinguished pups from harbor 

 seals, Phoca vitulina. No attempt was made to lo- 

 cate decomposed pups in the late breeding-season 

 photographs that were found earlier in peak breed- 

 ing-season photographs. Juveniles were yellowish in 

 color and generally were not found in the same vi- 

 cinity as the harems. Adult females were smaller 

 than adult males, were located within a harem, and 

 either attended a pup or appeared parturient. Sub- 

 adult and adult males were identified as non- 

 parturient animals with a penile opening or probos- 

 cis, or as nonparturient animals stationed along the 

 periphery of harems (see Le Boeuf, 1974; Cox, 1983; 

 and McLaren, 1993). Those that could not be classi- 

 fied were included in the "unknown" category. Adult 

 and subadult males were included in the same cat- 

 egory because we could not distinguish between 

 them. 



Our count of adult females may have contained 

 males that were three to seven years old that (on the 

 basis of our criteria) were not detected in large har- 

 ems. We believe that the probability of counting 

 males as females when the photographs were taken 

 is very low. Studies of northern elephant seals at Ano 

 Nuevo Island indicate that <1% of our count of adult 

 females could include young males. (Le Boeuf 2 ). 



The color-transparency photographs were illumi- 

 nated with a light table and inspected with a 7-70x 

 zoom binocular microscope. The seals were counted 

 according to each age and sex class category and their 

 numbers were tabulated on a transparent acetate 

 sheet when ten or more were in the photograph to 

 prevent under- or double-counting them. After count- 

 ing and marking the acetate sheet, the acetate sheet 

 was then placed on an adjacent photograph at the 

 exact location where the count terminated previously. 



2 Le Boeuf, B. J. 1994. Department of Biology and Institute of 

 Marine Sciences, Univ. California at Santa Cruz, CA 95064. Per- 

 sonal commun. 



