242 THE FUK SEALS OF THE PRIBILOF ISLANDS. 



uuniber which returu each year that from ouehalf to two thirds have perished before 

 the age of three years — that is to say, the kilhible age for the males and the breeding 

 age for the females. 



5. The chief natural ' causes of death among pups, so far as known at present, are 

 as follows, the importance of each being variable and more or less uncertain : 



(a) EaTages of the parasitic worm Uncinaria, most destructive on sandy breeding 

 areas and during the period from July 15 to August 20, 



(h) Trampling by lighting bulls or by moving bulls and cows, a source of loss 

 greatest among young pups. ' 



(c) Starvation of iiups strayed or separated from their mothers when very young 

 or whose mothers have died from natural causes. 



(f/) The ravages of the great killer (Orca), known to be fatal to many of the young 

 and perhaps also to older seals. 



At a later period drowning in the storms of winter is believed, but not certainly 

 known, to be a cause of death among the older pups. 



0. Counts of certain rookeries, with partial counts and estimates of others, show 

 that the number of breeding females bearing pups on St. Paul and St. George was, 

 in 1806 and 1897, between 160,000 and 130,000, more nearly approaching the higher 

 figure iu 1896 and the lower in 1897. ' 



7. On certain rookeries, where pups were counted in both seasons, 16,241 being 

 found in 1896 and 11,318 in 1897, or applying a count adopted by Professor Thompson, 

 11,713 in the latter year, there is evident a decrease of 9 or 12 per cent within the 

 twelvemonth iu question. The count of pups is the most trustworthy measure of 

 numerical variation in the herd. The counts of harems, and especially of cows 

 present, are much inferior in value. The latter counts, however, point iu the same 

 direction. The harems on all the rookeries were counted in both seasons. In 1896 

 there were 4,932; iu 1897 there were 4,418, a decrease of 10.41 per cent. The cows 

 actually present on certain rookeries at the height of the season were counted in both 

 seasons. Where 10,198 were found in 189G, 7,307 were found in 1897, a decrease of 

 28.34 per cent.^ 



8. It is not easy to apply the various counts in the form of a general average to 

 all the rookeries of the islands. We recognize that a notable decrease has been suffered 



' That is to say, uot inchidinj; losses ensuing from the killing of mothers at sea. 



The number of dead jiups counted on the rookeries between August 8 and 14, iu IStlG, was 11,045. 

 It is recognized that this number is au underestimate, inasmuch as a greater number must have been 

 overlooked than were counted twice. It is also recognized that the great majority of these jiups 

 died from the attacks of the worm Uncinaria. 



'The iniportance of this source of loss ^ve now find to be much less than was supposed to be the 

 case from the investigations made iu 1896. (See Keports for 1896, Jordan, p. 45; Thompson, p. 20; 

 Macoun, MSS.) 



^For detailed account of the census of 1896, see .Jordan, Preliminary Report for 1896, p. 15; 

 Thompson, Report for 1896, p. 19; Macoun, Report, 1896, MSS. For a discussion of suggested 

 corrections to the census of 1896, see Jordan, Final Report, 1897. For details of the census of 1897, see 

 Thompson, Report, 1897; Macoun, Report, 1897; Jordan, Report, 1897. A correction to be made in 

 the census of 1896 arises from the agreed assumption that the total number of breeding females was 

 1.75 times the number seen iu the height of the season. Later observations show that the actual 

 total is at least twice the maximum number ever seen at once on a rookery. 



<The extreme irregularity of the number of cows i)resent on the rookeries from day to day and 

 the consei|Ueut invalidity of any comparison of their number is shown by the counts madeou Lukauin 

 and Kitovi rookeries during the season of 1897. See Appendix II. 



