BREIWICK ET AL.: ESTIMATED INITIAL POPULATION SIZE OF BOWHEAD WHALE 



gram then calculates forward and adjusts the ini- 

 tial level until it yields the correct (i.e., assumed) 

 current stock size. Current population estimates 

 are based on sightings and therefore are presum- 

 ably for the total population. 



In order to reconstruct the 1848 level, it is neces- 

 sary to have a record of the catch history since that 

 time, together with a reasonable range of esti- 

 mates of the other parameters noted, i.e., mortal- 

 ity rate, maximum net recruitment rate, and cur- 

 rent population size. Estimated catches and other 

 assumed parameter values are discussed below. 



the known aboriginal catch iMaher and 

 Wilimovsky 1963; Durham 1979) with some addi- 

 tions (Marquette 1976, see footnotes 5 and 6; 

 Mitchell footnote 4 1 and adjusted by the struck but 

 lost (and assumed moribund) rates estimated by 

 Mitchell (footnote 4). These are summarized in 

 Table 1 (cf. Mitchell's table 9) for the entire period 

 1848-1977 (see footnotes under Mitchell's table for 

 discussion of extrapolations and modifications to 

 these data). 



Estimates of Current Stock Size 



METHODS 



Stock Size Analysis 

 Estimates of Initial Stock Size 



Rice (1974), using data of Clark (1887), esti- 

 mated a Bering Sea stock size of 4,000-5,000 ani- 

 mals during a peak harvest from 1868 to 1884. 

 Mitchell (footnote 4) concluded that the period of 

 peak catch was earlier and, after examining 

 methods of extrapolating catch from production 

 statistics (oil and baleen yield) and catch per ves- 

 sel, constructed a catch history (1849-1976) based 

 on these estimates or on known catch. Mitchell 

 summed the cumulative catch for the peak decade 

 (1851-60), applied a loss rate of 24^ , and concluded 

 that a minimum population of 11,647 bowheads 

 existed in 1850. He then performed the same 

 cumulative catch summation on what he termed 

 the "residual stock," which had survived, and 

 exploitation of which had resulted in another peak 

 catch period in the 1880's-1890's. 



In summing the two cumulative catch estimates 

 of population size, Mitchell corrected the latter by 

 subtracting from it an assumed net recruitment of 

 5% per year over the period between the peaks. He 

 concluded that the initial stock must have com- 

 prised approximately 18,000 bowhead whales. The 

 calculations discussed below are a refinement of 

 this rough procedure and show that this estimate 

 is not unreasonable. 



Catch History 



We have taken the best estimate of commercial 

 catch for each year or the known catch when avail- 

 able, and applied the struck but lost (and assumed 

 moribund) rates estimated by Mitchell (footnote 

 4). We have added to these commercial removals 



There have been few recent surveys or counts 

 which give quantitative estimates of total popula- 

 tion abundance. Counts, e.g., from ice edge sight- 

 ings through the season, were made by Braham 

 and Krogman,'' who estimated the 1976 inshore 

 migration from 25 April to 2 June to include 796 

 animals. Breiwick and Chapman^ extrapolated 

 these data to account for animals that migrated 

 earlier than 25 April or later than 2 June and 

 arrived at a total population of 1,227. However, a 

 more complete and careftil census was carried out 

 in 1978, in which whales were counted in the near- 

 shore lead as they passed Barrow, Alaska, between 

 15 April and 30 May. The estimate for this compo- 

 nent of the population was 2,264 (Braham et al. 

 1979). Aerial surveys were conducted in offshore 

 leads and no whales were observed. In the model 

 below we assumed 1978 stock levels of 900, 1,500, 

 2,100, and 2,700 animals. 



Vital Parameters 



As stated above, we assume a recruitment model 

 with appropriate parameter values. If natural 

 mortality is assumed to be fixed and the net re- 

 cruitment rate is a linear function of stock size, the 



^Marquette, W.M. 1976. National Marine Fisheries Ser\-ice 

 field studies relating to the bowhead whale harvest in Alaska, 

 1975. Processed Rep., 31 p. Natl. Mar. Mammal Lab., Natl. Mar 

 Fish. Serv., NOAA, 7600 Sand Pomt Way NE.. Seattle. WA 98115. 



«Marquette,W. M. 1978. The 1976 catch of bowhead whales 

 iBalaena mysticetus) by Alaskan Eskimos, with a review of the 

 fishery 1973-1976, and a biological summary of the species. 

 Processed Rep., 80 p. Natl. Mar Mammal Lab., Natl. Mar 

 Fish. Serv,, NOAA, 7600 Sand Point Way NE., Seattle, WA 98115. 



'Braham, H.W, and B.D.Krogman. 1977. Population biol- 

 ogy of the bowhead (Balaena mysticetus^ and beluga (Delphinap- 

 terus leucas) whales in the Bering, Chukchi and Beaufort 

 Seas. Processed Rep., 29 p. Natl. Mar Mammal Lab., Natl. Mar 

 Fish. Serv, NOAA, 7600 Sand Point Way NE., Seattle. WA 98115. 



^Breiwick, J. M., and D. G. Chapman. 1977, Population 

 analysis of the Alaska bowhead whale stock. Int. Whal. Comm. 

 Doc. SC SPC 13, 5 p. The Red House, Station Road. Histon, 

 Cambridge CB4 4NP Engl. 



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