POPULATION DYNAMICS AND EXPLOITATION OF SEALS 177 



high pregnancy rates (over 90 per cent) in the older adults and the short lag 

 (averaging less than a year) between first ovulation and first pregnancy in 

 newly matured females of that region both suggest that reproductive rates 

 are being maintained at a very high level. The sustainable annual kill of 

 7,000 seals would represent 7-10 per cent of a total population of 70,000- 

 100,000, while an independent population estimate based on fast ice distribu- 

 tion gives a closely comparable figure of 99,800 seals. Lastly, we may cite 

 information that the harp seal, which has much the same age of first reproduc- 

 tion, reproductive rates, and longevity as the ringed seal, is thought from 

 records of the effects of over-exploitation in the White Sea to have about 

 the same permissible catch of 8 per cent (Surkov, 1957; see also Sergeant, 



1959). 



While we have been dealing with the numbers of seals, yield is often best 

 dealt with in terms of biomass. In the first place there are considerable 

 geographical differences in the size of ringed seals (dealt with earlier). 

 Furthermore, with the increased mortality imposed by hunting there will be 

 an inevitable reduction in the average age, and hence size, of the seals. But 

 there is another curious property of these populations which must be 

 considered. If the age and size of the female seals have an effect in themselves 

 on the size of pups produced, then, through the mechanisms discussed earlier, 

 the age-specific size of seals may decrease with utilization. This is unlike the 

 common response of fish populations, where initial exploitation, at least, 

 often results in increased size of the fish. While at present we have empirical 

 figures on the average sizes of seals which may be combined with the 

 theoretically based maximum sustainable yields, more refined, dynamical 

 considerations of biomass yield must await further study. 



The bearded seal has unfortunately offered neither a completely suitable 

 method of age determination nor a sufficiently large series of specimens for 

 an analysis of sustainable yield. The female produces a pup only once every 

 two years, and this is a severe limitation. However, the collections available 

 do suggest that the species has a smaller mortality rate, for the ratio of 

 immature to adult seals is somewhat less than in the ringed seal. A maximum 

 sustainable yield of 5 per cent is suggested as a safe first approximation. 



AVAILABILITY OF SEALS 



Considerable discussion has been devoted to the subject of sustainable yields, 

 which may be of more general interest to the student o£ population equilib- 

 rium, liowever, the fact is that few regions in the Canadian north, to judge 

 from the catch statistics, are being over-hunted. Indeed, the economic 

 problem of most areas is not the sustainable yield of seals, but their availabihty 



