Crayfish Eradication for Sonora Mud Turtles

Introduction

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Virile crayfish (Orconectes virilis) are freshwater crustaceans that have a wide native range across eastern North America from the Missouri, upper Mississippi, lower Ohio, and Great Lakes drainages. Today, these crayfish are found in many locations outside their native range including other parts of the contiguous U.S., Mexico, and Europe. The use of crayfish as live bait by anglers has contributed to their introduction and spread within the U.S. Beginning in the 1950s, virile crayfish were stocked in the Colorado River watershed of western New Mexico and northeastern Arizona as a biological control for vegetation (Dean 1969). Since their establishment, crayfish have become invasive throughout much of Arizona’s aquatic systems. Virile crayfish may compete with and/or have contributed to the decline of native species in the Southwest, including the Little Colorado Spinedace (Lepidomeda vittata), Flannelmouth Sucker (Catostomus latipinnis), White Sands Pupfish (Cyprinodon tularosa), Chiricahua leopard frog (Lithobates chiricahuensis), three forks spring snail (Pyrgulopsis trivialis), and many other fish, macroinvertebrates, and macrophytes. Virile crayfish’s impact on surrounding biodiversity is directly attributed to predation and resource competition, such as displacing native species from shelter and overharvesting underwater plant beds. 

In 1996, biologists began studying aquatic amphibians and reptiles in Cottonwood Creek, a small tributary to Lake Pleasant in the Agua Fria River watershed in central Arizona, and initiated a 10-year population biology research project on the Sonora mud turtle (Kinosternon sonoriense). The area is characterized as a small Sonoran Desert stream with both permanent, interrupted water and intermittent flow that experiences seasonal drought often punctuated with periods of heavy rainfall and flooding. In recent years, Cottonwood Creek has suffered from extensive drought and native species declines, including its Sonora mud turtle population. The Sonora mud turtle is listed as a Species of Greatest Conservation Need in the Arizona State Wildlife Action Plan due to threats from bullfrogs, crayfish, and drought.

Key Issues Addressed

During their study at Cottonwood Creek, researchers observed negative impacts of crayfish on the native Sonora mud turtle population, including signs of predation on hatchlings. Researchers witnessed an adult crayfish repeatedly pushing a young mud turtle and found dead hatchlings with missing limbs and crushing and puncture injuries consistent with crayfish attacks. Researchers observed that crayfish prey on hatchlings and suspected that declines in mud turtles elsewhere were a result of crayfish predation. Virile crayfish are opportunistic feeders and the characteristics of turtle hatchlings, including their small size and lack of adequate shell calcification, make them attractive prey to adult crayfish. Sonora mud turtles are a long-lived, late-maturing species and researchers observed that the population within Cottonwood Creek had very low juvenile survivorship, most likely due to crayfish predation. As part of their overall research on this aquatic community, biologists and students immediately began opportunistically removing crayfish from the creek and ramped up eradication efforts in 1999. Researchers recognized the removal of the crayfish as a crucial step in helping to conserve the aquatic system’s native aquatic amphibians and reptiles, including the mud turtle population. 

Project Goals

• Examine the impact of introduced crayfish on the Sonora mud turtle population and test the hypothesis that crayfish can reduce or eliminate turtle recruitment
• Eradicate virile crayfish populations from Cottonwood Creek to conserve native aquatic species, including the Sonora mud turtle
• Demonstrate possible eradication methods for management of crayfish within similar environments

Project Highlights

Mudbug Roundup: The successful eradication of crayfish in Cottonwood Creek required a combination of methods: dip nets in shallow pools and hand capture under rocks and in bedrock crevices. 

• Opportunistic Efforts: Starting in 1996, biologists and their students removed crayfish throughout the study site whenever possible, but in 1999 began aggressively removing crayfish from the site. Data was not recorded for the total number of crayfish removed due to these eradication efforts being opportunistic, but one example included the removal of 164 crayfish in May of 2000. Removal efforts continued until researchers successfully achieved eradication in September of 2000. Biologists and students opportunistically visited the study site 402 times for Sonora mud turtle research between September 1996 and August 2005, with each visit averaging around 2-3 hours.
• Successful Crayfish Eradication:Crayfish failed to recolonize the creek for at least 4 years following eradication. Since the conclusion of the mud turtle research approximately sixteen years ago, researchers have only found a few crayfish within the lowermost reaches of Cottonwood Creek. Crayfish will likely continue to move upstream from Lake Pleasant when the creek flows, but they have failed to establish a permanent population or spread to the upper reaches of the watershed. The site has remained crayfish-free as of 2020.
• Immediate Mud Turtle Response:When crayfish were abundant, biologists never recaptured mud turtle hatchlings. Following eradication efforts, researchers documented the immediate benefits to the native mud turtle population. Among 29 hatchlings that were captured after crayfish eradication, fourteen were recaptured one or more times, demonstrating an immediate response to crayfish removal and directly improving mud turtle survivorship.
• Non-Native Aquatic Species Control Techniques: The characteristics of the study site allowed for researchers to successfully use dip nets to remove crayfish from pools that became small and isolated during drier times of year. Also, researchers captured crayfish by hand, often from beneath rocks or in bedrock crevices. Most crayfish were killed on site; a subset of specimens was preserved.

Lessons Learned

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The increase in hatchling recaptures immediately following crayfish eradication is strong evidence for increased mud turtle survivorship. Researchers did not keep detailed records of the crayfish numbers removed because it was largely an opportunistic effort that involved many individuals working independently. The small size of the stream allowed for researchers to successfully use dip nets and hand-capture. However, attempts to trap crayfish in minnow traps baited with liver were only marginally useful. It is important to note that the usefulness of these methods may differ in other environments.

When working with invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species in any environment, researchers emphasized the importance of persistence for management success. Eradication efforts must be aggressive and consistent for positive results to be seen. Knowing the species’ natural history is also vital to the success of invasive control to identify the species’ most vulnerable life history stage. For crayfish at this site, it appeared to be the stage just before breeding. By late summer and early fall, the population consisted of subadults and adults that were relatively easy to capture (compared to juveniles), and removing adults prevented the next year’s recruitment. Female virile crayfish can lay an average of 200 eggs from late May to early June. Removing the crayfish when they are at their most vulnerable, like many other invasive species, aids in effectively removing as many crayfish as possible before they have the opportunity to further overtake the system. Effective management of this site’s Sonora mud turtle population requires an understanding of their relationships between habitat use, demographics, and invasive predators.

Next Steps

• Continue crayfish removal whenever observed in Cottonwood Creek
• Apply these crayfish eradication methods to environments with similar landscape and small stream characteristics
• Communicate that crayfish control can be successful with sustained effort

Resources

• Article on Sonora mud turtles: Hensley et al. (2010). “Demography, Terrestrial Behavior, and Growth of Sonora Mud Turtles (Kinosternon sonoriense) in an Extreme Habitat.” Herpetological Monographs 24(1):174-193.
• USFWS Ecological Risk Screening Summary
• Arizona State Wildlife Action Plan

Contact

• Thomas R. Jones, Amphibians and Reptiles Program Manager, Arizona Game and Fish Department: tjones@azgfd.gov

CART Lead Author

• Ariana Porter, CART Student Writer, Northern Arizona University

Suggested Citation

Porter, A., C. (2021). “Opportunistic Crayfish Eradication Benefits Local Sonora Mud Turtle Population.” CART. Retrieved from https://www.fws.gov/project/crayfish-eradication-sonora-mud-turtles.