Diet and Body Mass of Ducks in the Presence of Commercial Harvest of Brine Shrimp Cysts in the Great Salt Lake, Utah

摘要

Commercial fisheries and waterbirds are often in competition for available resources and may affect each other through limiting available forage for birds or reducing harvest for commercial fisheries. Relatively little is known regarding the wintering population of waterfowl on the hypersaline Great Salt Lake (GSL), Utah, but mid-winter counts indicate waterfowl use the GSL during winter coinciding with a commercial harvest of brine shrimp (Artemia franciscana) cysts (i.e., eggs). We surveyed previously unmonitored pelagic regions of GSL to estimate total wintering populations of waterfowl and collected northern shoveler (Anas clypeata) and green-winged teal (Anas crecca) during 3 nonbreeding seasons, 2009-2012, to examine patterns in diet and body mass of birds using the GSL and estimate the amount of cyst removal by ducks. Our results indicated >200,000 northern shovelers and >150,000 green-winged teal wintered in the GSL and their diets, based on esophageal contents, changed seasonally, likely because of food availability. Esophagi contained predominately wetland plant seeds in fall and spring, whereas GSL invertebrates (i.e., brine shrimp adults and cysts, brine fly [Ephydra spp.] larvae) made up 75-100% aggregate wet weight during winter. Body mass of both species of ducks was greatest in fall and decreased through winter until a partial to full recovery of mass by spring. Though decreases in body mass were positively associated with decreases in GSL cyst abundance, cyst mass in duck diets peaked when cyst abundance was lowest in the GSL. Our results suggest current management of commercial cyst harvest leaves adequate cysts for waterfowl consumption during late winter. Furthermore, estimates of cyst removal from the GSL by ducks suggests they consume <1% of the yearly raw mass removed by ducks and commercial harvest combined, which implies ducks do not have an adverse impact on the commercial cyst harvest. We suggest managers begin incorporating pelagic areas in mid-winter waterfowl surveys to better monitor the relationship between cyst harvest and waterfowl abundance and distributions