The contribution of double-crested cormorants (Phalacrocorax auritus) to silver carp (Hypophthalmichthys molitrix) DNA loads in the Chicago Area Waterway System

Abstract

Abstract Waterfowl and colonial waterbirds can have significant impacts on water quality in lakes and reservoirs by depositing feces that contribute to nitrogen and phosphorus loads. Piscivorous birds can also contribute the DNA of prey species to a water body. Here, we develop and apply a loading model to estimate the number of silver carp (Hypophthalmichthys molitrix) DNA target marker copies that are potentially deposited by nesting double-crested cormorants (Phalacrocorax auritus) in the Chicago Area Waterway System (CAWS). The model assumes a conservative breeding population estimate ranging between 6000 and 8000 cormorants distributed among three large colonies in the Chicago metropolitan area. The model also assumes that cormorants are distributing feces randomly throughout the CAWS in proportion to the amount of time spent at each location. Results show that cormorants may be contributing 2.6 to 113 target marker copies/m2/day if birds are spending 22% of their time on open water and 6.4 to 291 target marker copies/m2/day if birds are spending 56% of their time on open water. Over the entire CAWS, cormorants may contribute tens of millions to billions of silver carp DNA copies each day. These target marker loads may be contributing to positive detections of silver carp environmental DNA (eDNA) in the CAWS. This study does not address other potential sources of silver carp genetic material in the CAWS, including live fish, and provides no indication as to whether or not the loads attributed to cormorants are large or small in relation to these other potential sources.