Lowry and Forney: Abundance and distribution of Zalophus californianus 



335 



it was assumed that the proportion of time spent at 

 or near the surface was similar for adult females and 

 other age and sex classes and did not vary significantly 

 within region, season, and year. 



All counts were conducted by the first author, who is 

 an experienced counter with high intercount reliability 

 (Lowry, 1999). Geographical positions (latitude and 

 longitude) were assigned to each haulout site. 



Photographic surveys 



The aircraft was flown from north to south directly 

 over the coastline or slightly offshore at an altitude 

 of 183 to 213 m (typically 213 m) to locate sea lions 

 onshore. The low altitude ensured that California sea 

 lions could be detected on rocky substrates, aided in 

 identification of different pinniped species, and enabled 

 accurate counts from aerial photographs. All hauled-out 

 California sea lions onshore were photographed. At the 

 Farallon Islands, the aircraft was flown at an altitude 

 of 366 to 457 m (typically 396 m) to prevent disturbance 

 of nesting seabirds. Multiple passes were made over 

 large rocks or islands to ensure that the entire rock or 

 island was photographed. Surveys were made without 

 regard to tidal conditions at any time of day between 

 approximately two hours after sunrise and two hours 

 before sunset. 



Sea lions were photographed with a 126-mm-format 

 KA-76 camera (Chicago Aerial Industries, Inc., Chi- 

 cago, IL) equipped with image motion compensation 

 (IMC) and operated at a cycle rate that achieved 67% 

 overlap between adjacent frames. The geographical 

 position of each photograph was recorded by linking 

 the camera (mounted vertically inside the belly of the 

 aircraft) to a computer and GPS unit. A 152-mm fo- 

 cal-length lens was used for low altitude photography 

 (i.e., 183-213 m) and a 305-mm focal-length lens was 

 used for higher altitude photography (i.e., 366-457 m). 

 Kodak Aerochrome MS Film 2448, a very fine-grained, 

 medium-speed, color transparency film, or Aerochrome 

 HS Film SO-359, a very fine-grained, high-speed, color 

 transparency film, was used. The camera was set at 

 an aperture of f/5.6 and a shutter speed between 1/400 

 and 1/2000 second. 



Photographic counts 



Sea lions were counted from photographs illuminated 

 with a light table by using a 7-30X zoom binocular 

 microscope. Counts were obtained for five age and sex 

 class categories: pups, juveniles, adult females or young 

 males of similar size, subadult males, and adult males. 

 Age and sex class distinctions were determined from 

 size and other external characteristics (e.g., hair color 

 on head, presence of sagittal crest, chest size, fore flip- 

 per width, snout shape, and body coloration). Animals of 

 each age and sex class were marked on a clear acetate 

 plastic overlay with different colored pens as each was 

 counted. Marks on the acetate were then compared and 

 verified with overlapping photographs. The acetate was 

 placed on another photograph at the exact position of the 

 coastline where the count ended previously and the count 

 was continued on the uncounted portion. One count was 

 made for each rock, island, or mainland haulout site. 



Analysis of haulout data 



Counts of sea lions made in this study were compared to 

 those obtained by earlier investigators in 1980-82 (Bon- 

 nell et al. 1 ) and 1995-96 (Beeson and Hanan 8 ) by using 

 nested ANOVAs and paired //-tests. The null hypothesis 

 of no difference in zonal counts was used to examine 

 differences in counts by zone, season, year, and survey. 

 The counts were 0.45 power transformed (with Systat 

 6.0 for Windows, SPSS Inc., Chicago, IL) because their 

 distribution was skewed toward zero. 



Results 



Sighting rates and g(Q) 



No difference was found (P>0.05) for number of sight- 

 ings, total animals seen, and mean group size during 

 Beaufort sea state conditions 1 through 4. A sharp 

 decline in sighting rates was observed when sightings 

 were grouped into glare categories of 0-24% (rc=27.3 

 sightings/1000 km), 25-49% (n = 17.5 sightings/1000 

 km), 50-74% (ra=10.7 sightings/1000 km), and 75-100% 

 (?2 = sightings/1000 km). Sighting rates were signifi- 

 cantly greater at 0-49% glare than at 50-100% glare 

 (P<0.001 for all surveys combined); therefore, only data 

 collected in 0-49% glare were used for at-sea abundance 

 estimation. With only data collected in 0-49% glare, 

 we used 48-76% of kilometers surveyed and 79-89% 

 of sightings. 



The probability of sighting a sea lion at the surface, 

 g(0), was estimated as 0.67 (with a CV of g(0) = 0.12). 



At-sea abundance 



Strip-transect survey effort totaled 1272 km during 

 26-30 May 1998, 2856 km during 12-28 September 

 1998, 2993 km during 1-11 December 1998, and 1175 km 

 during 13-21 July 1999 (Fig. 4). No transect survey was 

 conducted in July 1998 because of persistent low clouds 

 and high winds. Transect distances in 0-49% glare 

 conditions are given in Table 1. Nearly all sightings 

 were within the inshore stratum, and most were within 

 20 nautical miles from the mainland coast (Fig. 5). Cor- 

 rected at-sea abundance estimates for sea lions in the 

 study area (Table 1) were 28,340 (May 1998), 40,161 

 (September 1998), and 24,720 animals (December 1998). 

 For July 1999, a corrected abundance estimate for the 

 inshore stratum in July 1999 was 11,492 animals (Table 

 1). From the total abundance estimated in the three 

 1998 surveys, the average proportion represented by 

 the offshore stratum was 0.073 (range: 0.000-0.204). 

 From this proportion, we estimated that there were 

 about 829 sea lions in the unsurveyed offshore stratum 



