166 U. S. BUREAU OF FISHERIES 



COMPUTATION OF FUR SEALS, PRIBILOF ISLANDS, 1927 



By Edward C. Johnston 



The computation of the number of animals in the various classes in 

 the fur-seal herd was based on a count of the harem and idle bulls 

 present at approximately the height of the breeding season, a count 

 of the pups on one rookery, and data available from the work of 

 previous years. 



The count of the harem and idle bulls was made at about the same 

 time as in previous years. During the week following July 16, 

 annually, there are found the greatest number of bulls holding harems, 

 and it is during this period that they are enumerated. On St. Paul 

 Island, Lagoon, Tolstoi, Zapadni, Little Zapadni, Zapadni Reef, 

 Kitovi, and Lukanin rookeries were counted on July 16; Gorbatch, 

 Ardiguen, and Reef rookeries on July 17; and Polovina, Polovina 

 ClifTs, Little Polovina, Morjovi, and Vostochni rookeries on July 18. 



Fig. 17. — Group of fur seals, Pribilof Islands, Alaska 



On St. George Island, East Reef, and East Chffs rookeries were 

 counted on July 20; North and Staraya Artil rookeries on July 21; 

 and South and Zapadni rookeries on July 22. Sivutch rookery, 

 which is situated on a small islet off St. Paul Island, could not be 

 ■counted at the proper time on account of inclement weather. 



BULLS 



The idle-bull class showed a satisfactory gain in numbers in 1927. 

 The proportion of idle bulls to harem bulls should be between 20 and 

 25 per cent. The virgin cows do not come ashore until near the end 

 of the breeding season or at about the time the regular harem buUs 

 are ready to withdraw from the harems for their much-needed rest. 

 As the harem bulls leave the harems the idle bulls move in. There 

 are normally about one-fifth as many virgin cows as there are adults. 

 •Consequently, there should be one-fifth as many idle bulls as harem 

 bulls, and if it is considered that the period during which the virgin 



