POPULATION DYNAMICS AND EXPLOITATION OF SEALS 173 



Andrewartha & Birch, 1954), applies to the seal. There is, however, ample 

 evidence for several mechanisms which will combine to force an upper size 

 limit and a stable age structure on ringed seal populations. As has been 

 pointed out, surplus females may be forced to occupy less suitable ice, where 

 their pups may be precociously abandoned or even killed by unstable condi- 

 tions, and where exposure to predators may be greater. There is also evidence 

 that the pregnancy rate of adult females may be directly governed by the 

 availabihty of suitable ice. In areas of Baffin Island and Foxe Basin, where 

 there is much suitable ice and quite high hunting pressure, only one adult 

 female (pregnant at the end of March) has been taken in the total sample of 

 128 specimens from the open water outside the fast ice in late winter and 

 spring. As might be expected, almost all the adult females were successfully 

 engaged in reproduction within the fast ice, or at least were in a position to 

 be impregnated by the adult males which were also under that ice. In contrast 

 to this, near Churchill, on the open coasts of south-west Hudson Bay, where 

 there is little suitable ice and little hunting, only two specimens are available 

 from the open water in late winter; this would ordinarily be too small a 

 sample to tell us anything, but both seals were adult females, and neither 

 was pregnant ! There is no doubt that both sorts of limitation on the success 

 of reproduction are density-dependent, or compensatory, and will serve to 

 produce populations of equivalent size and age structure in regions with 

 equivalent amounts and quahties of fast ice. If territoriality per se is a factor 

 at all, it may apply more to the occupation of fast ice by adult males, which 

 often show severe bite-wounds in the breeding season. It has been imphed 

 that ringed seals are not limited by food supplies and indeed it is believed 

 that the control of populations through recruitment may preclude the action 

 of compensatory mortality on any age-group, except of course the pups. 

 However, there is good evidence of greater dispersal from regions of high 

 reproductive potential, and such emigration may be density-dependent. 



In order to calculate the size of seal populations, we may first assume that 

 equal areas of fast ice of the same type will contain the same number of 

 animals. Secondly we must discover how many seals occur in unit areas of 

 different kinds of fast ice. The quahty of fast ice doubtless depends on many 

 geographical and chmatic factors^, but coastal complexity is especially 

 important. An analysis which seems to give reasonable results at present 

 involves estimating the relative proportions of three arbitrarily-defmed 

 categories of ice — that within one mile of shore and surrounded by further 

 ice, that within one mile of land but exposed to open water on its seaward 

 edge, and that more than one mile from land. A few counts of seals lying on 

 the fast ice during the spring basking season suggest that these classes of ice 

 contain total populations of about thirty-five, ten, and five seals (or about 



