242 THE FUR SEALS OF THE PRIBILOF ISLANDS. 
number which return each year that from one-half to two thirds have perished before 
the age of three years—that is to say, the killable 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) Ravages of the parasitic worm Uncinaria, most destructive on sandy breeding 
areas and during the period from July 15 to August 20. 
(b) Trampling by fighting bulls or by moving bulls and cows, a source of loss 
greatest among young pups.” 
(c) Starvation of pups strayed or separated from their mothers when very young 
or whose mothers have died from natural causes. 
(d) 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. 
6. 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 1896 and 1897, between 160,000 and 130,000, more nearly approaching the higher 
figure in 1896 and the lower in 1897. ° 
7. On certain rookeries,where pups were counted in both seasons, 16,241 being 
found in 1896 and 14,318 in 1897, or applying a count adopted by Professor Thompson, 
14,743 in the latter year, there is evident a decrease of 9 or 12 per cent within the 
twelvemonth in 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 in the same 
direction. The harems on all the rookeries were counted in both seasons. In 1896 
there were 4,932; in 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 1896, 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, not including losses ensuing from the killing of mothers at sea. 
The number of dead pups counted on the rookeries between August 8.and 14, in 1896, was 11,045. 
It is recognized that this number is an underestimate, inasmuch as a greater number must have been 
overlooked than were counted twice. It is also recognized that the great majority of these pups 
died from the attacks of the worm Uncinaria. 
2The importance of this source of loss we now. find to be much less than was supposed to be the 
case from the investigations made in 1896. (See Reports 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 
correctious 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 in 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 present on the rookeries from day to day and 
the conseyuent invalidity of any comparison of their number is shown by the counts made on Lukanin 
and Kitovi rookeries during the season of 1897. See Appendix II. : 
