Model-Based Predictions of the Effects of Harvest Mortality on Population Size and Trend of Yellow-Billed Loons
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Resource Abstract:
Yellow-billed loons (Gavia adamsii) breed in low densities in northern tundra habitats in Alaska, Canada, and Russia. They
migrate to coastal marine habitats at mid to high latitudes where they spend their winters. Harvest may occur throughout the
annual cycle, but of particular concern are recent reports of harvest from the Bering Strait region, which lies between Alaska
and Russia and is an area used by yellow-billed loons during migration. Annual harvest for this region was reported to be
317, 45, and 1,077 during 2004, 2005, and 2007, respectively. I developed a population model to assess the effect of this
reported harvest on population size and trend of yellow-billed loons. Because of the uncertainty regarding actual harvest
and definition of the breeding population(s) affected by this harvest, I considered 25 different scenarios. Predicted trends
across these 25 scenarios ranged from stability to rapid decline (24 percent per year) with halving of the population in 3
years. Through an assessment of literature and unpublished satellite tracking data, I suggest that the most likely of these
25 scenarios is one where the migrant population subjected to harvest in the Bering Strait includes individuals from breeding
populations in Alaska (Arctic coastal plain and the Kotzebue region) and eastern Russia, and for which the magnitude of harvest
varies among years and emulates the annual variation of reported harvest during 2004-07 (317, 45, and 1,077 yellow-billed
loons). This scenario, which assumes no movement of Canadian breeders through the Bering Strait, predicts a 4.6 percent rate
of annual population decline, which would halve the populations in 15 years. Although these model outputs reflect the best
available information, confidence in these predictions and applicable scenarios would be greatly enhanced by more information
on harvest, rates of survival and reproduction, and migratory pathways.
Citation
Title Model-Based Predictions of the Effects of Harvest Mortality on Population Size and Trend of Yellow-Billed Loons