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AUTHORS: Catherine A. Richter, Katy E. Klymus, Nathan Thompson, Duane C. Chapma - U.S. Geological Survey
ABSTRACT: Use of environmental DNA (eDNA) to detect rare or invasive species is complicated by the indirect nature of eDNA detections. In particular, eDNA monitoring cannot distinguish between different sources of DNA such as live and dead fish. This aspect of eDNA monitoring has caused concern in the Chicago Area Waterway System (CAWS), which is being actively monitored for two species of invasive Asian carp, silver carp and bighead carp. Although both eDNA and traditional monitoring have shown the CAWS to be upstream of the current invasion front, carp monitoring efforts in the CAWS are challenging because there are many invaded water bodies nearby, as well as fish markets selling silver and bighead carp in Chicago. Thus, human activity and fish-eating wildlife could potentially deposit carcasses and other traces of carp in the CAWS and create an eDNA signal. In order to quantify the contribution of eDNA from carcasses, we compared the eDNA signal from silver carp carcasses with that from live fish stocked at two different densities in outdoor mesocosms. Experimental ponds with approximate volumes of 260 m3 were stocked with no fish, 10 silver carp with a total biomass of approximately 12 kg, 100 silver carp with a total biomass of approximately 110 kg, or one previously frozen silver carp carcass of approximately 2.5 kg. The carcass was placed in a wire mesh cage to prevent uncontrolled scavenging, and one live crayfish was added to the cage at the start of the trial. This experimental setup was repeated for three trials, and eDNA was sampled before adding fish and every 24 hours after adding fish for 7-14 days. We found that the eDNA signal from the single carcass was approximately equivalent to the eDNA signal from 100 live fish, and lasted at least 9 days post-stocking.