Table 4. — Numbers of sea lions, Bristol Bay district, 1956-57 



determined from photos 



[* indicates visual estimate] 



Rookery and 

 code numbers—' 



Adults Pups Total Adults Pups Total 



1956: 



1. Cape Newenham. 



2. Twin Island. . . 



Total 



Survey 1, July 26- Aug. 4 

 250 250* 



300 300* 



1957; 



550 



550 



Survey 6, Sept. 10 

 30 0- 30* 



147 147 



177 







177 



1/ Code numbers refer to map in figure ID. 



and exposure of photographs between the 

 hours of 1000 and 1600. The counts, therefore, 

 reflect a period when both the behavior pattern 

 of sea lions and the effect of weather condi- 

 tions tended to stabilize and maximize the 

 counts. 



Nevertheless, the results of surveys made 

 on closely spaced trips or on successive days 

 differed substantially. The differences appear 

 to be erratic and may be caused by sea lions 

 moving from land to sea or from one island 

 to another. In addition, certain age groups, 

 such as immature males, may not frequent 

 the rookeries at all during the breeding 

 season. This appeared to be the case on 

 Chernabura Island in 1958.' The length fre- 

 quency distribution of males shows no im- 

 mature males after the yearling stage. 



Seasonal Variations 



From a cursory inspection of tables 1 to 6, 

 it is evident, either from the counts obtained 

 on individual rookeries or from the total 

 count of a district, that great changes took 

 place in the number of sea lions on a rookery 

 during the year. Generally the counts were 

 low in the early spring, reached a maximum 

 in the late summer months, and declined again 

 toward the end of the year. 



' See footnote 1 on p. 6. 



To estimate seasonal changes, counts from 

 successive surveys of 18 major rookeries 

 were summarized for 1956 and 1957 (table 8). 

 On survey 2 in 1957 some of the large rooker- 

 ies were not covered, and estimated figures 

 were used. These were derived from the 

 arithmetic mean of the counts made on sur- 

 veys 1 and 3 in the same year or the nearest 

 adjacent surveys. 



Two of the three districts, which extend 

 from Cape St. Elias to False Pass (fig. la, 

 lb, and le), show the same general fluctua- 

 tions, with a build-up in population density 

 during the breeding season in June. The 

 summer populations were commonly from 

 two to three times larger than the winter 

 ones (fig. 6). 



Two explanations appear possible; many 

 of the animals may migrate from the area, 

 or while foraging in the sea they may spend 

 increasing amounts of time away from the 

 rookeries. 



Since no winter survey was made of rook- 

 eries west of Unimak Pass, it is not known 

 if there is a migration in that direction. It 

 seems unlikely, because the winter weather 

 is more severe to the west. Probably the 

 observed decline during winter is simply the 

 result of more intensive feeding and longer 

 journeys from the rookeries and hauling areas. 



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