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/ 



Fur seal pups sleeping on the sand near Tolstoi Rookery, St. Paul Island, 

 in late July. These pups, which are less than a month old, are resting during 

 one of the 5- to 14-day intervals between nursings. 



Mortality and Disease 



When the Pribilof herd was small, about 2 

 percent of the pups died on land; now from 

 10 to 15 percent are dead before the autumn 

 migration. In each of recent years, from 40,000 

 to over 100,000 have died on the Pribilof Islands. 

 More than half perish from the effects of hook- 

 worm infection. Starvation, physical injuries, 

 congenital defects, and bacterial infections also 

 kill many. No epizootic diseases have been ident- 

 ified, although the sudden death of several hund- 

 red seals on the Commander Islands in 1958 and 

 off St. George Island in 1962 places suspicion on 

 a disease or food toxin. 



Hookworms occur in the tissues of seals of all 

 ages. The parasites are apparently able to pen- 

 etrate the skin, at least on bare surfaces such 

 as the flippers. Pups are infected through 

 nursing, and the parasite attaches principally to 

 the small intestine. Bloodsucking by the worm 

 causes severe anemia. By fall, the intestinal in- 

 fection disappears, but hookworms are retained 

 in the blubber, mammary tissue, and perhaps 

 other tissues. There is no evidence that adult 

 worms from the intestines move into the blubber. 

 Larval worms that penetrate the skin hatch from 

 eggs passing from the intestine of pups. Reasons 



11 



