THE SEA OTTER IN THE EASTERN PACIFIC OCEAN 143 



(51°28' N. lat, 179°11' E. long,) to (but not including) East 

 Cape (51°22'32'' N. lat., 179°2r45'' E. long.) The surveys were 

 conducted in February, March, and April of 1959 (see Kenyon and 

 Spencer, 1960). 



These surface surveys differ from the others because an inten- 

 sive effort was made in a limited area to observe, by repeated trips, 

 the available sea otter habitat at close range in locations familiar 

 to the observer. The mean number of otters observed was 254. On 

 an aerial survey of the same section of coast in May 1959, 192 

 otters were counted. Thus, approximately 76 percent of the number 

 seen from the surface was observed from the air. 



If only the highest surface counts for the section of the coast 



are considered, the total count for the area is 295 otters. This 



figure would indicate that on the aerial survey about 65 percent 



4.4.1, 4.4. • ,192X100, 

 01 the otters m the survey area were seen ( — ^95 — )• 



Based on the premise that the maximum number of otters seen 

 on surface surveys failed by 15 percent to include all those present, 

 an aerial survey would reveal 50 percent of the otters present. This 

 correction factor was used previously (Kenyon and Spencer, 1960). 



The correction factor indicating that about 75 percent of the 

 otters were seen is a conservative one and is used to estimate total 

 populations in the following discussion for these reasons: (1) Tag 

 recoveries at Amchitka revealed that the home range of a sea otter 

 may include about 10 miles of coastline. It therefore appeared 

 reasonable to use the mean of counts for each part of the area 

 studied. In this way some compensation is made for the movements 

 of individual otters in and out of the study areas during intervals 

 between surveys. (2) Many observations during aerial surveys, 

 when the same locations were flown over repeatedly, indicated to 

 us that, in general, under excellent conditions we saw at least 75 

 percent of the otters present. In some places, having a minimum 

 number of rocks and kelp breaking the surface, where the shore- 

 line was even and the band of water furnishing available feeding 

 habitat was narrow, we felt reasonably certain that, in calm 

 weather, we saw more than 75 percent of the otters present. 

 (3) We found that water depths as shown on USCGS charts con- 

 stituted a dependable guide to locations inhabited by sea otters. 

 A large majority occur within the 20-fathom curve (waters less 

 than 120 feet in depth). Rarely do sea otters occur in water as 

 deep as 30 fathoms. Therefore we consider that no significant 

 number of sea otters was missed during the aerial surveys. 



Future studies may show that we were unable to estimate the 



