352 



Alaska — Our Liviiii; Re si in ires 



Fig. 2. Apprcixiniate bounds of 

 the Beaufort Sea and Chukchi Sea 

 polar bear populations. The con- 

 tours for each population surround 

 959c and 5(Y'c of the radio reloca- 

 tions that were nearest the har- 

 monic mean center of the distribu- 

 tion of relocations. 



Fig. 3. Curves comparing survival 

 of male and female polar bears in 

 early (1967-74) and late (1981-92) 

 periods. Note poorer survival of 

 young and improved survival of 

 adults, indicative of a relatively 

 large population, in later period. 



Beaufon Sea. however. We compared 986 cap- 

 tures and recaptures from the 1967-74 period to 

 1.331 captures and recaptures from the 1981-92 

 period to evaluate population trends. 

 Reproduction among females commonly began 

 at age 6 and continued until at least age 24. 

 Numbers of cubs produced per female in both 

 time periods were similar, but litter sizes of 

 yearlings were larger in the first period. 

 Differences in sampling during the two periods 

 may have prevented effective comparisons of 

 birth rales and of litter sizes; the age structure of 

 the population was younger in the first period. 

 Survival of adults, as calculated from life tables, 

 was higher and survival of young lower in the 

 1981-92 period (Fig. 3). Radio-collared bears 

 had a survival rate of 0.965 (96.-3% survived), 

 and their dependent young survived at the rate 

 of 0.676 (67.6% survived). Of 26 radio-collared 

 females followed until death. 22 (84%) were 

 shot by coastal hunters. 



We used a modified Petersen mark-and- 

 recapture model (Seber 1973) to estimate there 

 were approximately 600 females in the 

 Beaufort Sea in 1976. Placing our calculated 

 birth and death rates into matrix models pro- 

 jected growth to 900 females and 1.500 total 

 animals in 1992. This was a realized growth 

 rate of about 2% per year. The modified 

 Petersen model provided an estimate of 750 

 females for 1986. That growth rate projected 

 forward to 1992 indicated 850 females and just 

 over 1.400 total animals; numbers that agreed 

 closely with those predicted by the matrix mod- 

 els. 



Numbers of bears captured per unit of effort. 



in the Beaufort Sea, also have increased, pro- 

 viding another indication of population growth. 

 The few catch/eHbrt data from the Chukchi Sea 

 also suggest an increasing trend. There was a 

 compensatory relationship between estimated 

 population size in the Beaufort Sea and recruit- 

 ment of subadults. Large populations of recent 

 years recruited few juveniles, and smaller pop- 

 ulations present in the first period recruited 

 higher proportions of juveniles. 



Implications of Growth 



We are confident that the growth we detect- 

 ed in the Beaufort Sea population is real. A 

 finite rate of growth of 19^-2% and a cunent 

 population of approximately 1 .500 are both rea- 

 sonable. Increased numbers of polar bears seen 

 along Alaska's north coast in recent years. 

 increased encounter rates by researchers, and 

 matrix models all suggest the population is larg- 

 er now than in the recent past. This increase in 

 numbers has occurred despite continued hunt- 

 ing by local resident Native people, and despite 

 development of the nation's largest oilfield at 

 Prudhoe Bay. The age structure and survivor- 

 ship patterns of recent years suggest the popula- 

 tion in the Beaufort Sea may be at or near the 

 limits set by its environment. 



Unfortunately, known and unknown biases 

 in our mark and recapture data resulted in pop- 

 ulation size estimates that were associated with 

 considerable uncertainty. The degree of fluctua- 

 tion we observed in population estimates 

 derived by the sophisticated Jolly-Seber model 

 were biologically impossible. The estimates 

 were more consistent in the simpler Petersen 

 model, substantiating the observation that the 

 trend of increase is valid, but not erasing con- 

 cerns about the absolute size of the population. 

 Less-than-perfect population estimates may not 

 be an urgent problem if harvest is kept at a level 

 that is known to be within long-term sustained 

 yield (e.g.. near present harvest levels). 

 Hunting, however, already accounts for 80% of 

 calculated annual mortality, and pressures to 

 increase harvest are always present. Estimates 

 of the size of the population of polar bears in the 

 Chukchi Sea are lacking, but the catch per unit 

 of effort during research tagging there may sug- 

 gest an increase, as do observations and kills by 

 coastal residents. Uspenski and Belikov (1991) 

 believe there are more bears in the Chukchi Sea 

 now than in the past despite the absence of a 

 reliable population estimate. 



Thus, the good news of apparent increases in 

 numbers is accompanied by increased chal- 

 lenges for management. Those challenges can 

 only be met by a better understanding of the 

 dynamics of the polar bear's ecosystem. In the 



