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Century-long impacts of increasing human water use on numbers and production of the American White Pelican at Pyramid Lake, Nevada

Publication Type:

Journal Article

Source:

WATERBIRDS, Volume 28, p.61-72 (2005)

Abstract:

Anaho Island, Pyramid Lake, Nevada supports the largest colony of the American White Pelican (Pplecanus erythrorhynchos) in western North America. Counts of adults, nests and chicks, from 1903 to the present, are positively correlated with flow volumes of the Truckee River in spring near Nixon, Nevada, 15 km from the lake. Upriver diversions of water have markedly reduced flows of the lower Truckee River and have likely depressed pelican production; for example, the Derby Dam, completed in 1906, diverts water from the Truckee River into the Truckee Canal and out of the Truckee River basin. American White Pelican production was negatively correlated with flows in the Truckee Canal, If water had remained in the river instead of being diverted, it is estimated that annual colony production would have averaged 8% higher. A more complete model, accounting for seasonal diversions of water from the river between the gage upriver at Farad and the gage near Nixon, predicted that colony production would have averaged as much as 58% higher if natural flow regimes had persisted in the Truckee River downriver from Farad. Human removals of water from the Truckee River, and thus simulated reductions on pelican colony production, were highest in drought years. Despite the likely reductions in production throughout the last century, particularly in drought years, numbers of adults and nests at this breeding colony demonstrated no long-term directional trends. Therefore, reductions in production apparently have been offset by higher survival or immigration from other colonies. Even so, higher production could be an important buffer against high mortality events, such as the disease die-offs of large numbers of American White Pelicans at Salton Sea, California, and even natural disasters described by other authors in this symposium.