174 I- A. McLaren 



seven, two, and one birth lairs) respectively per square mile of ice. The esti- 

 mates may have more relative than absolute vahdity, for they assume that the 

 entire population may be counted on the fast ice at the height of the basking 

 season. The extent of fast ice has been measured from maps, and population 

 estimates made for different sections of coast-lines. A total population of 

 close to a million is suggested by these means for the eastern Canadian Arctic 

 south of Lancaster Sound and north of Cape Chidley and James Bay. 



It has been suggested that populations of under-utilized bearded seals may 

 be Hmited by the accessibility of shallow, ice-free feeding banks in winter. 

 Unfortunately, suitable hydrographic charts are very few, and we cannot 

 even begin to develop a key to the numbers of bearded seals comparable to 

 that developed for the ringed seal. The only way to derive a first approxima- 

 tion of numbers at present is to compare the relative abundance of this species 

 and the ringed seal in shipboard censuses (actually through means of the 'index 

 of availabUity' outlined below) and extrapolate to coast-lines which are 

 generally similar in shallowness and ice cover. 



SUSTAINABLE YIELD OF SEALS 

 If the seals of a region are to support Eskimo needs indefinitely, then the 

 mortality imposed by hunting and natural causes must not exceed the 

 recruitment into the population. 



On the Hudson Strait coast of Baffm Island, from which we have the 

 largest collections, the catch statistics suggest that seal numbers have remained 

 steady in recent years. Local or seasonal shortages or increased catches may 

 safely be attributed to local changes in hunting pressure, weather or economic 

 factors. But we have no a priori reason for presuming that the population is 

 being kept in balance by hunting mortahty, for it has been shown that 

 under-exploited populations will have their equilibrium maintained by ice 

 conditions. Future work on populations experiencing various levels of 

 hunting mortaUty is planned to assess its effect on pupping success, pregnancy 

 rates, emigration, and natural mortality. 



If we assume that the equilibrium population of southern Baffm Island is 

 in fact experiencing maximum possible hunting mortaUty, the result should 

 be to give us a very safe estimate of the sustainable yield from an analysis 

 of the available mortality and reproductive rates. 



Where Xj is the number of seals in age-group j at the beginning of the 

 seal year, where Sj is the annual survival rate of the age-group j, where Bj 

 is the birth rate of the age-group j, and where there is no reproduction by 

 the first or last (Z*^) age-groups, and no emigration or immigration, then 

 in an equilibrium population: 



Xo = B^SoX^ + BAXi + . . . 5z5^e_i) X(^_i) (i) 



