FISHERY BULLETIN: VOL. 81, NO. 2 



the last two censuses are a result of limited light, then 

 our estimates for these 2 yr are biased downward, but 

 only by a small amount. 



Because the night rate is the single largest ex- 

 trapolation of the estimation procedure, more direct 

 evidence on this would be highly desirable. Perhaps 

 radiotelemetric studies in progress by Mate and Har- 

 vey (1979) 5 will help to clear up remaining ambiguity 

 on this point. 



The mean estimated group size increased with in- 

 creasing distance from shore. This prompts two vary- 

 ing interpretations: This result may be an accurate 

 depiction of whale behavior, or it may be an indica- 

 tion of greater sightability of larger groups farther 

 offshore. The correction used here for whales missed 

 offshore is based upon the assumption of equal 

 sightability of groups, independent of group size 

 within 1.5 mi (2.4 km), during periods of unhampered 

 visibility. If the distribution of group sizes is in fact 

 uniform with respect to shore, and small groups are 

 missed near the outside of the 1.5-mi (2.4 km) zone, 

 our population estimates would be biased down- 

 ward. 



Even after correction for varying amounts of poor 

 visibility conditions and proportions of the popula- 

 tion missed offshore, there is a considerable amount 

 of year-to-year variation within the significant in- 

 crease noted here. This may be due to further effects 

 of visibility conditions or to unaccounted variation 

 between counters. It also may be due, in part, to vary- 

 ing proportions of the population overwintering 

 north of the Monterey area during different years. An 

 investigation into the possible relationships of the 

 changes in migratory timing to seasonal environmen- 

 tal events in the Arctic Ocean and North Pacific is in 

 progress and may help clarify this problem. 



The annual estimates presented here are slightly 

 higher than those reported earlier (Reilly et al. 1 980), 

 especially for years with many days of poor visibility, 

 primarily due to correction for this factor. The 

 variances presented are also of a slightly greater 

 magnitude than those previously reported. These are 

 probably a more realistic representation of the varia- 

 tion inherent in the estimates, because they now in- 

 clude consideration of variation from both the effect 

 of visibility conditions, and the inconsistency of es- 

 timating distances to passing whales. The general 

 magnitude of the derived variances was independent- 

 ly corroborated by the subsampling exercise. 



-Mate, B. R., and J. Harvey. 1979. A successful new radio tag for 

 large whales. [Abstr.] 3d Biennial Conf. Biol. Mar. Mamm., Seattle, 

 Wash. (Available from B. R. Mate, Marine Science Center, Oregon 

 State University, Newport, OR 97365.) 



Regarding the current population level, we have 

 produced three estimates: 17,577 from the latest 

 census, 16,186 extrapolated from a simple linear 

 model of increase, and 15,647 extrapolated from the 

 weighted log model of increase. The 95% confidence 

 intervals of all three overlap the point estimates. 

 Given the range of extrapolations employed, the 

 most conservative route is to choose the lowest, 

 15,647, as the "best" estimate of current popula- 

 tion size. 



A statistically significant increase in population 

 level of about 2 .5 %/annum was calculated from these 

 census results. If one also considers the annual har- 

 vest of about 164 whales by the Soviet subsistence 

 fishery near the Chukotski Peninsula (Ivashin and 

 Mineev 1978; International Whaling Commission 

 1979), the total net annual rate of production was 

 probably near 3.75% for the past 13 yr. To our 

 knowledge, this is the first empirical substantiation 

 of a net increase in size by a whale population which 

 was under exploitation. 



ACKNOWLEDGMENTS 



During 1978-79, Geoffrey Carrol and Steven 

 Savage manned the counting station; Melvin Jasper 

 piloted the aircraft; Alan Baldridge, Nancy Barnes, 

 Nancy Belser, Howard Braham, Maurice Malhki, 

 Leslie Reilly, Vicki Smith, Richard Strassel, and 

 Laurie Stewart recorded data during flights. During 

 1979-80, Gerald Joyce and Bruce Krogman manned 

 the station; Lawrence Johnson piloted the plane; 

 Gerald Freidman, Gerald Joyce, Lynn Moses, 

 Madeline Moses, Randall Puckett, Leslie Reilly, and 

 Eric Sund recorded data during flights. Douglas 

 Chapman gave statistical advice. Leslie Reilly helped 

 compile aerial data; Muriel Wood and Mary DeWitt 

 typed various drafts of the manuscript. John 

 Kuhlmann and Chris Bouchet gave computer pro- 

 gramming advice and aid, and their efforts are 

 gratefully acknowledged. Special thanks to Michael 

 Tillman of the National Marine Mammal Laboratory, 

 for making this analysis possible, Michael Tillman, 

 David Rugh, Howard Braham, Shari Sitko, and 

 Roger Pearson read earlier drafts and contributed 

 valuable suggestions. Thanks are also due to an anony- 

 mous reviewer for comments and suggestions which 

 clarified the presentation of quantitative material. 



LITERATURE CITED 



Allen, K. R. 



1980. Conservation and management of whales. Univ. 

 Wash. Press, Seattle, 107 p. 



278 



