104 ALASKA FISHERIES AND FUR INDUSTRIES IN 1917. 



AGES OF COWS. 



rt is a well-established fact that the female seal begins her breeding 

 career when 2 years old and brings forth her first pup when 3 years 

 old. The male, however, is unable to breed under normal conditions 

 until he is 7 years old. The length of the breeding period of both 

 these classes is a very important matter and unfortunately is not 

 very well known. It can only be learned from branded animals, and 

 the number which can be thus marked is manifestly an insignifu ant 

 portion of the total. For several seasons, the avera;2,e breeding period 

 of the cow has been placed at 10 years. 



In 1900, 1901, and 1902 a number of pups were branded with a 

 bar across the middle of the back. Some of these have returned an- 

 nually ever since, and a photograph of one was printed in the report 

 of the fur-seal investigation in 1914."^ Altogether five were seen in 

 1917. This is very significant because the time given to search for 

 them was exceedingly limited. It demonstrates, howevo", beyond 

 question that the female does live 15, 16, or 17 years; that is, she can 

 have 12, 13, or 14 pups. From this it appears that the deduction of 

 10 per cent from the breeding-cow class each year for old-age mor- 

 tality is entirely sufficient. 



THE 3-YEAR OLD COWS. 



It is well to again test the natural mortality of the seals by applying 

 the . knowledge we have of the breeding cows, the same as has been 

 done for two years past. The total number of breeding cows in 1916 

 was 116,977. By deducting the 10 per cent loss from old-age mor- 

 tality, as explained above, of these in 1917 there should remain 

 105,280. To this number there was an increment of 22,744 composed, 

 of 3-year-old females born in 1914, which brings the total up to 128,024. 

 It is easy to ascertain the loss of female pups born in 1914 which 

 would leave these 22,744. The total number of pups born that year 

 was 93,350, half of which, or 46,625, should have been females. If 

 the losses for the first three j^ears had been exactly 50 per cent there 

 would have been an increment of 23,312 in place of the 22,744, which 

 has been computed. This is as close as a calculation of this character 

 could be expected to come. It is entirely possible in one case that 

 the births of males exceeded the females in 1914 by 568, which would 

 make up the difference. And again it is possible that the loss varies 

 to a certain extent each year and may sometimes run a little under 

 or over this 50 per cent. At any rate this seems a very safe basis to 

 work from and has been used in all deductions from all classes for 

 natural mortality during the first three years of the seals' lives. 



THE 5-YEAR-OLD BRANDED COWS. 



Little can be said about the cows which were branded as pups 

 in 1912 and were 5 years old in 1917. They were seen on practi- 

 cally every rookery and throughout the breeding season. The ani- 

 mals appeared normal in every respect and the brands seen were 

 very distinct. Compared with unbranded cows on the rookeries 

 they appeared very young. Wliile they had not in most cases 

 acquired the complete mask of white whiskers, they were much more 



• Bureau Fisheries Document No. 820, PI. XII. 



