Green Sunfish: Aquatic Gremlins of the Southwest

Authors: Eric Frye, David Ward

Year: 2022

Abstract

The 1980’s Steven Spielberg movie, “Gremlins”, featured a small invasive creature brought home as a pet, which subsequently reproduces, terrorizes the town, and eats its inhabitants. Green sunfish (Lepomis cyanellus) in a similar fashion do not reach large body sizes, do not have large teeth or noticeably large mouths, and are commonly transported into new locations by unsuspecting adults and children. They quickly reproduce and aggressively prey on other fishes, eliminating recruitment of southwestern native fishes in waters where they coexist. Despite their small size, and relatively benign appearance, green sunfish arguably do more damage to native fish populations than any other invasive fish in the desert southwest. When flood events occur, they spread rapidly across the landscape. In 2021, at Palmeroy Tanks, near Williams, Arizona, we recorded downstream movement of green sunfish before and after monsoon flooding. Thousands of individuals exited an existing pond and moved over 10 kilometers downstream during a single rain event to colonize a series of previously dry ponds. In the laboratory, we evaluated the predation potential of green sunfish in 24-hour trials and compared their predation ability to other introduced predatory fishes found in the Colorado River Basin. Green sunfish commonly consume juvenile chub at a larger proportion of their maximum anatomical gape (50%) than even smallmouth bass. Their aggressive predatory behavior at extremely small sizes allows them to access larval native fish populations in shallow, near-shore environments that are not available to other predators. Green sunfish then continue to prey upon native fishes until the prey outgrow the maximum anatomical gape of about 35 mm for adults. The high propensity of green sunfish to be moved by humans and then dispersed during flood events, their ability to survive and reproduce under a wide range of environmental conditions, and their aggressive predatory behavior makes them a significant threat to native fish conservation in the desert southwest.