Spatial and temporal distribution of phalaropes (Phalaropus spp.) and adult brine flies (Ephydra spp.) are linked on Great Salt Lake, Utah.

Great Salt Lake (GSL), Utah, is home to some of the world’s largest concentrations of Wilson’s Phalaropes (Phalaropus tricolor), Red-necked Phalaropes (Phalaropus lobatus), American Avocets (Recurvirostra americana), and Black-necked Stilts (Himantopus mexicanus). These birds spend several weeks on GSL feeding on larvae, pupae, and adult brine flies (Ephydra spp.) before migrating to their wintering grounds in Central and South America. Unfortunately, GSL is shrinking in size and becoming more saline due to water diversions and climate change. Assessing how a smaller and more saline GSL will affect adult brine flies is difficult without knowing their temporal and spatial distribution on GSL. During 2014 and 2015, we measured adult brine fly abundance across different GSL bays during July through September, when phalaropes are staging on the lake. Abundance of adult brine flies showed a bimodal pattern, peaking during weeks 27–29 and then again during week 33. Wilson’s Phalaropes were abundant during the first peak in fly numbers but not during the second peak; Red-necked Phalaropes exhibited the opposite pattern. Time of day and weather had little impact on adult fly numbers, with the exception that abundance was positively correlated with air temperature. During this study, a deep brine layer (which is toxic to brine fly larva) existed at water depths >6 m, and adult brine fly abundance was low over these areas. Adult brine fly abundance did not differ with distance from shore or time of day but was positively correlated with salinity. Substrate impacted their abundance, with adult brine flies being 5 times more abundant over microbialites—which are rocky, reef-like structures—than over sand or mud. Phalarope numbers were also highest over microbialites. Thus, most brine fly eggs are laid over microbialites, which is the preferred habitat for brine fly larvae and pupae. Information about the temporal and spatial distribution of adult brine flies is critical for wildlife managers who are tasked with ensuring that GSL will continue to support the large avian populations that depend on GSL’s adult brine flies for their existence. [ABSTRACT FROM AUTHOR]