Fall Migration of the Northern Shoveler at Hypersaline Mono Lake, California

From 2002 to 2022, waterfowl were surveyed at Mono Lake to evaluate the their response to lake-level changes under the State Water Resources Control Board’s landmark Decision 1631, in which the public trust doctrine was first applied to water rights. From September through mid-November, the total numbers of Mono Lake’s most numerous dabbling duck, the Northern Shoveler (Spatula clypeata), averaged 13,018 ± 1571 (standard error), ranging from a high of 27,400 in 2008 to a low of 2719 in 2018. Annual peak counts averaged 5562 ± 709, ranging from a low of 768 in 2018 to the highest single-day count of 13,793 in 2012. Annual totals were higher under conditions of monomixis than under the stratified condition of meromixis. In a generalized linear model, the combined effects of survey period, biomass of Mono Lake brine shrimp (Artemia monica), and regional drought explained 67.8% of the variation in Northern Shoveler numbers at Mono Lake, i.e., higher Artemia biomass and more severe drought in northeast California resulted in higher shoveler counts. Lake level, a key component of waterfowl-habitat restoration, showed no effect within the range of lake levels observed. These results suggest that food resources and regional drought have been the most important factors influencing Northern Shoveler use of Mono Lake, so far overriding any small-scale influences of habitat variation due to lake-level changes.