260 



NORTH AMERICAN FAUNA 74 



than 6% per year. Krylov (1963) estimated the recruitment rate of Pacific 

 walruses to breeding age at 6.3% per year; Fedoseev and Gol'tsev (1969) esti- 

 mated that recruitment did not exceed 9% per year. 



Other gregarious and polygynous pinnipeds, such as the gray seal (Halichoerus 

 grypus), elephant seal {Mirounga spp.), and northern fur seal, show survival of 

 young during the first year ranging from about 40 to 85% (Hewer 1957, 1964; 

 Laws 1960; Chapman 1964; Mansfield and Beck 1977), apparently being highest 

 when population density is low. Survival during the second (70 to 86%) and 

 third years (80 to 90%) tends to be still higher. Survival rates in those species 

 generally are highest (85 to 90 % ) over the next 5 or 6 years, then decline steadily 

 thereafter. Survival of males tends to be slightly lower than that of females in the 

 first 6 to 8 years and to be much lower thereafter. 



I assume that specific mortality of walruses changes with age in a manner 

 comparable with those of the other polygynous pinnipeds. Certainly, the mor- 

 tality from intraspecific traumatization is greater in calves than in any of the 

 older age classes (Fig. 129). Mortality of calves also is higher than that of nearly 

 all other age classes in the harvests. Furthermore, calves seem to be the primary 

 targets of natural predators, such as killer whales and polar bears; probably they 

 also are more subject to disease and thermal stress than the older animals. 



Some indication of the mortality of calves in the perinatal period (April-June) 

 is provided by data obtained by J. J. Burns and me from the Eskimo's harvests at 

 St. Lawrence and Little Diomede islands in spring 1952-68. Of 148 fertile 

 females that had recently given birth, 135 (91 %) were accompanied by healthy 

 calves; 13 (9 % ) had lost their calves at or soon after birth. Although this suggests 

 that about 9% of the calves had died, we found at the same time seven other 

 calves with foster mothers. If some of those were the lost calves, then the propor- 

 tion that died may have been less than 9% . In a sample from the Chukchi Sea in 

 July-August 1978, A. A. Kibal'chich (unpublished data) found 40 (34.5%) of 

 116 adult females accompanied by calves of the year. The predicted mean 

 number of calves born to a sample of this size (116 x 0.367) would be 42 to 43, 

 suggesting that about 5 to 7 % mortality had taken place in the first 2 to 3 months 

 after birth. The perinatal mortality of young of other polygynous pinnipeds 

 evidently varies widely in relation to population densit>', being as low as 2 to 3% 

 in populations that are still in the rapid growth phase, and as high as 13 to 60% 

 in those near maximum size (Laws 1953^?; Kenyon et al. 1954; Summers et al. 

 1975; Le Boeuf and Briggs 1977). 



In their estimate of population composition, Fedoseev and Gol'tsev (1969) 

 assumed that the survival of walrus calves to 1 year of age was between 65 and 

 73% by analogy with other pinnipeds, but they believed that the actual rate 

 might be somewhat higher. Chapskii (1936) estimated from the calf:cow ratio of 

 a nonselective harvest sample taken in the Kara Sea that the survival of young 

 from birth to 2 years was about 80 to 82 % . My results from two small samples 

 counted in the Bering Sea (Table 50) lend support to Chapskii's (1936) estimate, 

 but the sample size is too small for reliability. 



The underrepresentaton of animals 1 to 6 or 7 years old in both the harvests 

 and the natural mortality from intraspecific trauma (Figs. 127, 129) suggests 

 that the specific survival rates of those animals probably are the highest of any 

 age classes. Survivorship appears to decline markedly after about 8 to 10 years of 



