170 I. A. McLaren 



The bearded seal, on the other hand, feeds almost entirely on large bur- 

 rowing or sedentary bottom invertebrates (these species are not taken by the 

 ringed seal) and is not found in deep water. It also differs from the ringed 

 seal in being generally incapable of keeping open holes in the thick, land- 

 fast ice in the winter. It seems most abundant in the Arctic where reasonably 

 shallow banks are free of fast ice in winter, and equilibrium populations may 

 be determined by the availability of food, although the nature of this 

 dependence is not yet understood. 



Both species show a decline in feeding activity in the spring, which is the 

 moulting and basking season. The blubber of an average ringed seal drops 

 from a winter peak of about 40 per cent of the animal's weight to a summer 

 low of about 23 per cent. This blubber loss results in decreased buoyancy 

 and a much greater proportion of seals killed in the water by rifle fire in 

 early summer sink and are lost. 



Our understanding of reproduction in these species is enhanced by reliable 

 age determinations. Previous estimates of the age of newly-matured seals 

 have usually given too low a figure, and this of course introduces a dangerous 

 error into determinations of the rational exploitation level. In the ringed seal 

 there are about 46-5 females to 53 • 5 males. The males of both species usually 

 mature when seven years old. The females generally mature (first ovulation) 

 in their sixth or seventh year, but in the ringed seal, for which we have much 

 more information, they do not always become pregnant from the first 

 ovulation. In south-west Baffin Island about 10 per cent of ringed seals 

 produce a pup when six years old, about 50 per cent when seven, and about 

 75 per cent when eight. Almost 100 per cent of older females are fecund, the 

 proportion of successful pregnancies probably decreasing slowly with age so 

 that an average pregnancy rate of 90-95 per cent is established. We have 

 little information about the post-reproductive period, but this is believed to 

 begin in the late twenties or early thirties; very few females reach these 

 ages. It should be pointed out here that population control of the ringed 

 seal is beheved to act at least partly through reproduction, so that the age- 

 specific birth rates given above cannot be universally apphed. 



Both ringed and bearded seals have delayed implantation of the blastocyst, 

 which has given rise to false assumptions about the breeding seasons. The 

 males of both species are in full rut in the spring, from just before until 

 shortly after the females give birth to the pups. The female ringed seal 

 ovulates at this time and is impregnated while still nursing her pup. Those 

 bearded seals which have given birth apparently fail to ovulate until after the 

 rut of the males is over, and thus a two-year cycle of pup production is 

 estabhshed. 



The bearded seal lives in the open water the year round and breeds on the 



