Although environmental DNA (eDNA) is increasingly being used to survey for the presence of rare and/or invasive fishes in aquatic systems, the utility of this technique has been limited by a poor understanding of whether and how eDNA concentrations relate to fish density, especially in rivers. We conducted a field study to systematically test whether the eDNA released by a model invasive fish, Silver Carp (Hypophthalmichthys molitrix), was related to the density of this species in a large river. We quantified fish density throughout the 460 km long Illinois River using hydroacoustic surveys at 23 sites while concurrently collecting 192 surface water samples for eDNA analysis. We found that Silver Carp numerical density and biomass density were positively and non-linearly related to eDNA concentration and detection rate. Both eDNA concentration (copy number) and detection rate increased rapidly as Silver Carp density increased but plateaued at moderate densities. These relationships could prove useful for estimating Silver Carp relative abundance in newly invaded locations where population numbers are low to moderate. Future studies should explore the causes of this nonlinear relationship as it would ultimately benefit aquatic species monitoring and management programs.
Bibliographical noteFunding Information:
Funding was provided by the Great Lakes Restoration Initiative, via the Illinois Department of Natural Resources (Grant CAFWS-123A; https:// www.dnr.Illinois.gov) to DPC, AAC, and JEG. Funding was also provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative- Citizen Commission on Minnesota Resources (LCCMR; https://www.lccmr.leg.mn) to PWS. The Minnesota Aquatic Invasive Species Research Center (https:// www.maisrc.umn.edu) administered LCCMR funds. The funders provided support in the form of salaries for authors [DPC, PW, AAC, GEV, JJE, PWS], but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ?author contributions? section. Following completion of this study, JJE became employed by Fisher Scientific Inc. Fisher Scientific Inc. had no official collaboration with this study, did not influence study design, data collection and analysis, decision to publish, or preparation of the manuscript. We thank Kevin Irons, Matt O?Hara, Tristan Widloe, and Greg Whitledge for logistical support and Andrea Lubejko, Justin Seibert, Jennifer Heim, and Jen-Luc Abeln for field assistance and data processing. The Illinois Department of Natural Resources and the Illinois Natural History Survey provided fish catch data.
© 2019 Coulter et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.