Figure 2.- -Part of sea lion rocks adjacent to Pye Island in the Prince William Sound district. The counted portion 

 is outlined, and the overlap is determined in conjunction with the adjacent frame seen in figure 3. 



A 35-mm, camera with interchangeable lens 

 and automatic film advance was used during 

 most of the surveys discussed in this paper. 

 It combined compactness and ease of rapid- 

 sequence exposure and was far more satis- 

 factory than an aerial camera (k-20) that 

 was used occasionally. Usually all pictures 

 were exposed through an open window in the 

 plane's cockpit. A 90-mm. lens was most 

 satisfactory, but it was occasionally replaced 

 with a 50-mm. or a 135-mm. lens to meet 

 changing flying conditions. Today several other 

 types of cameras that offer a choice of 

 negative sizes and high-speed, long focus 

 lenses are available for rapid-sequence 

 photography. 



Exposure time was usually 1/1000 of a 

 second and seldom more than 1/500 of a 

 second. Slower exposures invariably resulted 

 in blurred pictures that were difficult or 

 almost impossible to count with any degree 

 of precision. 



Ordinarily, black and white film with an 

 ASA rating of 80 was used, but under ad- 

 verse light conditions it was replaced by 

 film with an ASA rating of 200. Films were 

 developed in Kodak D-76 or with Ethol ultra- 

 fine grain developer, and glossy prints, 8 by 

 10 inches, were prepared of the negatives. 



An attempt was made to expose negatives 

 with a 25-percent overlap wherever there was 

 a continuous distribution of sea lions. En- 

 largements could then easily be matched and 

 the best portion of each print counted. 



Categories of Animals 



Four categories of animals were recorded: 

 (1) adults on rocks, (2) adults in the water, 

 (3) estimated animals, and (4) pups. 



The first two categories represented all 

 sea lions that clearly could be counted from 

 the photographs on land or in the water. A 



4 



