PHOTOGRAPHIC CENSUS OF THE STELLER 

 SEA LION HERDS IN ALASKA, 1956-58 



by 



Ole A. Mathisen, Research Associate Professor 



and 



Ron J. Lopp, Assistant Fisheries Biologist 



Fisheries Research Institute, College of Fisheries 



University of Washington, Seattle, Washington 



ABSTRACT 



An aerial photographic technique for censusing herds of Steller sea lions 

 (Eumetopias juhata) in Alaska is described. The minimum number of sea lions from 

 Cape St. Elias to the Islands of the Four Mountains was estimated to be about 

 110,000, based on photographic censuses of rookeries and hauling grounds in 1957. 



The heaviest population density was recorded in an area between the entrance 

 of Cook Inlet and Unimak Pass. Pronounced seasonal variations were observed, 

 with a peak population on the rookeries from July to September. 



A partial aerial photo census of the harbor seals (Phoca vitulina) in Alaska is 

 discussed in the appendix. 



INTRODUCTION 



The problem of predatlon on salmon is 

 acute in Alaska today because many salmon 

 runs have declined to only a fraction of the 

 level of 20 to 30 years ago. Stringent re- 

 strictions placed upon the fishing industry 

 during the last decade to increase escape- 

 ment have failed to stem this serious decline. 

 Since Steller sea lions (Eumetopias jubata) eat 

 salmon as well as halibut and other types of 

 fish, it is only natural that they have been 

 suspected as a contributing cause to the 

 decline of the commercial salmon fisheries. 

 Accusations against sea lions as consumers 

 of food fish have at times been violent but 

 generally without a background of demon- 

 strable facts. Even today, a basis for estimating 



the possible gain in the monetary value of 

 the salmon runs versus the cost of controlling 

 the number of sea lions does not exist. One 

 of the steps necessary to make such an esti- 

 mate is to assess the number of animals in 

 different herds. This report describes the 

 development of an aerial photo census tech- 

 nique and results of surveys made during 

 1956, 1957, and 1958. 



Work on a census of sea lions was started 

 in 1952 by W. F. Thompson of the Fisheries 

 Research Institute and consisted of mapping 

 rookeries and hauling grounds in theShumagin 

 and Kodiak Island areas and experimenting 

 with aerial photographs as a basis for popula- 

 tion estimates (Thompson, Thorsteinson, and 

 Bevan, 1955). This work was supported by the 



