Defending Crystal Spring: eradicating the green sunfish (Lepomis cyanellus) from Ash Meadows National Wildlife Refuge

Authors: Emma Priger, Michael Reeves, Leah Simantel

Year: 2022

Abstract

Ash Meadows National Wildlife Refuge (AMNWR, Refuge) in southeastern Nevada is the largest remaining oasis in the Mojave Desert, supporting at least 50 springs and seeps within approximately 24,000 acres. These springs and seeps support the second highest level of endemism in North America, with 26 known rare and endemic species, including 3 endangered, endemic fishes. In January of 2021, nonnative green sunfish (Lepomis cyanellus) were found in the Crystal Spring system, the largest spring on the Refuge, presumably having invaded from a downstream reservoir after the failure of a fish barrier. Crystal Spring is a popular visitor destination and important habitat for several endemic aquatic species including the Ash Meadows Amargosa pupfish (Cyprinodon nevadensis mionectes) and several unique invertebrates including the Crystal Spring springsnail (Pyrgulopsis crystalis), making it a priority for aquatic invasive species control. A green sunfish control effort was initiated in February of 2021 to reduce competition and predation pressures on these endemic species, focusing on mechanical control methods over chemical methods to limit non-target mortality. The control effort was designed within an adaptive management framework, incorporating effectiveness monitoring in the form of catch and effort statistics for individual gears and system-wide monthly visual snorkel surveys. Results are being used to reallocate effort among gears and locations to maximize catch. Rod and reel angling, unattended angling (i.e., yo-yos), spearfishing, and hoop nets have been used, yielding varied results. Hoop nets have yielded the largest catch across all methods (n=178). Rod and reel angling (n=12) and spearfishing (n=12) have also been successful, particularly in specific habitats such as the spring head. Hoop net catch rates and visual snorkel surveys have both been encouraging, with the number of sunfish detected declining over time, presumably in response to declining abundance. The integrity of downstream fish barriers is also being monitored as a hedge against reinvasion using both visual inspections and a stream-wide PIT tag reader to detect green sunfish previously tagged downstream (n=21). There have been no detections of tagged fish above existing fish barriers, verifying their integrity to date. Refuge staff continue to adapt control efforts with the goal of complete green sunfish eradication prior to 2024 when a large-scale restoration of the lower Crystal Spring outflow is scheduled to commence.