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 15th to August 20th.
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. 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.
The causes of death here enumerated are natural and inherent in the conditions under which the herd exists. That some of them were not known or fully understood until the investigations of 1896 and 1897 does not make them new or recent in their action. They have been constant factors, acting with greater intensity in the past when the herd was larger and more crowded upon its breeding grounds. Photographs taken in 1891 and 1892 show that the parasitic worm was then doing its deadly work, and more extensively in proportion as the herd was larger. For 1,495 pups dead from this cause counted by us on Tolstoi sand flat in 1896, 4,000 were counted on the same ground by the British commissioner of 1892. Moreover, the bones of innumerable pups on ground already abandoned in that year by the declining herd attest the existence of this cause of death prior to that time. We have no reason to suppose that it has not always preyed upon the herd. Death by trampling must at present be at a minimum on account of the scattered condition of the rookeries. The storms of winter and pelagic enemies must, of course, take toll in proportion to the number of animals.
But the significant fact shown by this proposition is that the gain of the herd must be small at best under such a natural death rate. We may suppose these natural losses to have been the checks which in a state of nature prevented the indefinite increase of the herd. When, therefore, to this total loss of from two thirds to four fifths of the entire birth rate before breeding age is attained, we add the tremendous artificial loss through the destruction of gravid and nursing females resulting from pelagic sealing, it is not to be wondered at that the equilibrium was broken and the herd sent on a rapid decline.
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 Island was, in 1896 and 1897, between 160,000 and 130,000, more nearly approaching the higher figure in 1896 and the lower in 1897.
These figures are based upon counts of all the breeding families on both islands for each season. On certain rookeries the live and dead pups were counted. In this way an average size of family was obtained which was used to complete the census where pups could not be counted.
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 nine to twelve 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.
The important element in these special counts, undertaken with a view to determining the relative condition of the breeding herd for the two seasons, is the count of pups. All other classes of rookery population fluctuate from day to day, but the pups remain constantly on shore and near to the place of birth for the first six weeks of their lives, and it is merely a matter of patience and skill in counting them. Such a count on any rookery is an absolute record of the number of breeding females which has visited it for the season in question.
The minimum figure of nine per cent adopted by Professor Thompson is based upon a recount of a single rookery made by himself under conditions less favorable for accuracy than in the case of the official counts, which give the larger figure of twelve per cent, and which were made jointly by representatives of both commissions.
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 by the herd during the twelvemonth 1896 to 1897, without attempting, save by setting the above numbers on record, to ascribe to the decrease more precise figures.
This is a rather extreme statement of the uncertainty which may be assumed to attach to these figures. The problem is not an easy one at best and its factors are complex. This should always be borne in mind, but not to the extent of doubting the value of the figures. The areas counted were large enough to be fairly typical. The counts were carefully done, and are accurate enough for all practical purposes. The probable error for the 15,000 more or less pups counted would not exceed 500. But as the counting was done in exactly the same manner and by the same persons for the two seasons, such errors as may exist are common to both counts and the relative conditions are unaltered. The figure of twelve per cent, moreover, must be taken as in itself a minimum, since it is the result of a number of individual counts varying in accuracy; and all in a sense underestimates, inasmuch as more animals are always overlooked among the rocks than are counted twice.