NOTE Lowry et al : Counts of Mirounga angustirostns from aerial photographs 



179 



a; 



t 



Elephant seals were counted in this 

 manner until the entire photo- 

 graphed coastline was examined. 



A subjective evaluation was used 

 to determine the degree of survey 

 coverage after the elephant seal 

 count for each island was com- 

 pleted. A "total" rating was assigned 

 when all sites of the rookery were 

 photographed or visually inspected. 

 A "near-total" rating was assigned 

 when small portions of the rookery 

 were not photographed (the number 

 of seals missed was estimated to be 

 equal to or less than 1% of the total 

 counted). A "partial" rating was as- 

 signed when >\% of the total 

 counted were assumed not to have 

 been photographed. Total counts of 

 seals for surveys with a partial rat- 

 ing are normally not provided. How- 

 ever, three partial surveys, two at 

 San Miguel Island in 1985 and one 

 at San Nicolas Island in 1992, were 

 augmented with counts from pho- 

 tographs taken from high altitude 

 ( 1985) or from the previous peak 

 breeding-season survey (1992). 



Precision 



Independent counts were made of 

 elephant seals in photographs 

 taken at San Miguel Island, the 

 largest northern elephant seal rook- 

 ery (Stewart et al., 1994), to exam- 

 ine the precision of the counts ob- 

 tained from aerial photographs. 

 Three counts were made for each of 

 the two 1988 surveys and four 

 counts were made for each of the 

 two 1989 surveys. Two persons con- 

 ducted the counts in 1988 and three 

 persons in 1989 (for each year, one 

 person made two counts for each sur- 

 vey, and the other person! s ) made one 

 count). The coefficient of variation 

 (CV) of replicate counts was used to compare preci- 

 sion in the counts of all age and sex class categories 

 of seals. Precision was rated excellent, good, accept- 

 able, or unacceptable according to CV's obtained by 

 ground counts at Piedras Blancas (described below). 

 The following ratings, and criteria for defining each 

 rating, are given as follows: 1) excellent when the 

 CV was less than one-half the smallest CV obtained 



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Figure 1 



(A) Northern elephant seals, Mirounga angustirostris, at Point Bennett, San 

 Miguel Island, California. The photograph was taken with a 126-mm-format 

 camera with IMC from an altitude of 259 meters (850 feet). IB) Enlarged inset 

 from (A). These photographs display the following: 1) gulls (probably western 

 gulls, Larus occidentalis) among the elephant seals; 2 1 decomposed carcasses 

 of pups; 3) pups; 4) adult males; 51 subadult males within the harem; 6) adult 

 females with a pup; 7) adult females that appear parturient; 8) California sea 

 lions. Zalophus californianus; and 9) tracks on the sand made by elephant seals. 



by ground counts; 2) good when the CV was between 

 the smallest CV of ground counts and half that value; 

 3) acceptable when the CV equaled those obtained 

 by ground counts; and 4) unacceptable when the CV 

 exceeded the largest CV obtained by ground counts. 

 The counts of elephant seals by people on the 

 ground were compared with counts made from 126- 

 mm-format aerial photographs to ascertain the pre- 



