Potential population-level effects of increased haulout-related mortality of Pacific walrus calves
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Availability of summer sea ice has been decreasing in the Chukchi Sea during recent decades, and increasing numbers of Pacific walruses have begun using coastal haulouts in late summer during years when sea ice retreats beyond the continental shelf. Calves and yearlings are particularly susceptible to being crushed during disturbance events that cause the herd to panic and stampede at these large haulouts, but the potential population-level effects of this mortality are unknown. We used recent harvest data, along with previous assumptions about demographic parameters for this population, to estimate female population size and structure in 2009 and project these numbers forward using a range of assumptions about future harvests and haulout-related mortality that might result from increased use of coastal haulouts during late summer. We found that if demographic parameters were held constant, the levels of harvest that occurred during 1990–2008 would have allowed the population to grow during that period. Our projections indicate, however, that an increase in haulout-related mortality affecting only calves has a greater effect on the population than an equivalent increase in harvest-related mortality distributed among all age classes. Therefore, disturbance-related mortality of calves at coastal haulouts may have relatively important population consequences.
KeywordsBering Sea Chukchi Sea Haulout Mortality Odobenus rosmarus divergens Pacific walrus Population dynamics Projection model Subsistence harvest Trampling
We thank the U.S. and Russian subsistence hunters who provided samples and data used to estimate harvests. We also appreciate helpful discussions with B. Taras about the original values used in the Fay et al. (1997) walrus population model. The manuscript was improved as a result of helpful comments by J. MacCracken, N. Roberts, and three anonymous reviewers. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of the U.S. Fish and Wildlife Service. This article has been peer reviewed and approved for publication consistent with USGS Fundamental Science Practices (http://pubs.usgs.gov/circ/1367). Any mention of trade names is for descriptive purposes only and does not constitute endorsement by the federal government.
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