Potential of environmental DNA to evaluate northern pike (Esox lucius) eradication efforts: An experimental test and case study

Abstract

Determiningthe success of invasive species eradication efforts is challenging because populations at very low abundance are difficult to detect. Environmental DNA (eDNA) sampling has recently emerged as a powerful tool for detecting rare aquatic animals; however, detectable fragments of DNA can persist over time despite absence of the targeted taxa and can therefore complicate eDNA sampling after an eradication event. This complication is a large concern for fish eradication efforts in lakes since killed fish can sink to the bottom and slowly decay. DNA released from these carcasses may remain detectable for long periods. Here, we evaluated the efficacy of eDNA sampling to detect invasive Northernpike (Esox lucius) following piscicide eradication efforts in southcentral Alaskan lakes. We used field observations and experiments to test the sensitivity of our Northernpike eDNA assay and to evaluate the persistence of detectable DNA emitted from Northernpike carcasses. We then used eDNA sampling and traditional sampling (i.e., gillnets) to test for presence of Northernpike in four lakes subjected to a piscicide-treatment designed to eradicate this species. We found that our assay could detect an abundant, free-roamingpopulation of Northernpike and could also detect low-densities of Northernpike held in cages. For these caged Northern pike, probability of detection decreased with distance from the cage. We then stocked three lakes with Northernpike carcasses and collected eDNA samples 7, 35 and 70 days post-stocking. We detected DNA at 7 and 35 days, but not at 70 days. Finally, we collected eDNA samples ∼ 230 days after four lakes were subjected to piscicide-treatments and detected Northernpike DNA in 3 of 179 samples, with a single detection at each of three lakes, though we did not catch any Northernpike in gillnets. Taken together, we found that eDNA can help to inform eradication efforts if used in conjunction with multiple lines of inquiryand sampling is delayed long enough to allow full degradation of DNA in the water.