YORK and KOZLOFF: NORTHERN FUR SEAL PUPS 



about 5.5% per year (Fowler 1985). It is possible that 

 entanglement in debris was indeed responsible for 

 the decline during 1975-81, however, the data in 

 Figure 4 do not seem to support this hypothesis. If 

 entanglement were the principal cause of this 

 decline, we would have expected the population to 

 have continued to decrease at the pre- 1981 rate 

 since the observed entanglement rates have re- 

 mained stable since 1976. 



We may never know the cause of the 1975-81 

 decline in northern fur seal production. In general, 

 estimates of population size are highly variable so 

 several censuses are required to detect a statistically 

 significant decrease in a population; thus, the fact 

 of a decline, unless it is sudden and dramatic, is not 

 usually known for several years following its initia- 

 tion. Post facto studies are invariably subject to 

 criticism for flaws in experimental design; thus, 

 careful continual monitoring of the many aspects of 

 the biology of a population is the best hope for 

 ascribing a particular cause to a population change. 

 Comparisons of the population dynamics, food 

 habits, incidence of diseases, and entanglement 

 rates of northern fur seals with other pinniped 

 species which share their habitat in the North Pacific 

 Ocean might shed additional light on the various 

 hypotheses. 



ACKNOWLEDGMENTS 



We thank R. H. Lander for suggesting this study 

 and for indicating the value of the relationship 

 between numbers of breeding males and numbers 

 of pups. Review of this manuscript and helpful sug- 

 gestions for its improvement were provided by H. 

 Braham, R. L. DeLong, C. W. Fowler, G. Y. Harry, 

 Jr., D. Kimura, R. H. Lander, and R. Pearson. 



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