Razorback Suckers in Lake Mead

Introduction

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Lake Mead is the largest reservoir not only on the Colorado River, but it is also the biggest in North America in terms of water capacity. Lake Mead was created in 1936 by the completion of the Hoover Dam (originally named Boulder Dam). At maximum capacity it is 112 miles long, up to 532 feet deep, and can hold 28 million acre-feet of water. This man-made lake is located on the border of Arizona and Nevada and it is a popular spot for outdoor recreation activities, including boating, kayaking, and fishing. Home to many species of introduced sport fish such as Striped Bass, Largemouth Bass, and Catfish, Lake Mead also provides habitat to one of the few fish native to the Colorado River, the Razorback Sucker (Xrauchen texanus).

Razorback Suckers are immense, long-lived fish, growing to lengths of over three feet and some reaching 40 years of age. The Razorback Sucker, like all Colorado River native fish, evolved in a warm, turbid environment with few natural predators. These charismatic catastomids (sucker fish), listed under the Endangered Species Act in 1991, are known for their distinct keel behind their heads. Razorback Suckers were detected during surveys of Lake Mead until the 1970s, when the species was thought to have become locally extinct. A surprise sighting of an adult Razorback Sucker eating popcorn at a boat launch, by a Lake Mead visitor, occurred in the 1990s. Yearly surveys are conducted to monitor the status of the Lake Mead Razorback Suckers, and results now indicate that the population is self-sustaining. This has important implications for maintaining native fish species in the Colorado River Basin because the species is not self-sustaining elsewhere in the system.

Key Issues Addressed

The Razorback Sucker, like all Colorado River native fish, evolved in a warm, turbid system where they had few natural predators. The construction of dams, and the creation of large reservoirs that release cool, clear water, combined with the introduction of piscivorous non-native fish, caused a decline of once-common Razorback Suckers. Adult fish can persist in these new water conditions and flow regimes, but larval fish are often victims of predation. In many remaining populations of Razorback Suckers, the problem is not that the adults do not spawn, but that the larval fish do not recruit (survive to adulthood). Given this lack of recruitment, Razorback Sucker populations must be supplemented with hatchery-reared adults. These fish are past the delicate juvenile stage, so they are more likely to survive and join the reproducing population. Adult Razorback Suckers spawn each spring, and biologists have discovered that in Lake Mead, sucker larvae survive into adulthood. Nevertheless, investigations continue to explore why some populations have successful recruitment in locations that contain aquatic predatory species. Increased understanding of the conditions leading to survival of Razorback Sucker fry (young fish) in Lake Mead could be used to improve conservation and recovery of this species across the Colorado River Basin.

Project Goals

• Identify the spawning location(s) of adult Razorback Suckers in Lake Mead using various sampling methods and technologies
• Determine and quantify the ecological conditions required for successful spawning 
• Better understand the biological and ecological conditions that promote larval Razorback Sucker survival, while populations elsewhere in the Colorado River Basin require supplementation from fish hatcheries
• Use data collected from long-term surveys to directly inform management actions and habitat restoration for Razorback Suckers 

Project Highlights

Follow the Leader: Biologists are tracking acoustic-tagged adult Razorback Suckers in Lake Mead to learn more about their movement, habitat use, and spawning behavior at various spawning grounds.

• Collaboration: Yearly monitoring is a large-scale endeavor that requires many hands and funding from multiple sources. The Razorback Sucker population in Lake Mead would not be so well-investigated without the help of public and private partners including both funding from agencies within the Lower Colorado Multi-Species Conservation Program and contractor support.
• Genetic Management Tools:Multiple genetic monitoring tools are used to manage genetic variation, avoid genetic bottlenecking events that may decrease fitness, and to detect fish presence in the absence of traditional detection. Using DNA from wild-caught fish, development of eDNA markers for the Razorback Sucker is currently underway which can be used to determine the presence of the species without a visual detection, beneficial in areas where traditional survey methods are difficult to conduct. 
• Follow Razorback Sucker Adults: In order to determine the spawning locations of adult Razorback Suckers in Lake Mead, a small group of acoustic-tagged Razorback Suckers is used to locate schooling adults. Yearly monitoring is implemented in which multiple crews employ techniques such as trammel netting and collection of larval samples to confirm spawning activity at the locations of the tagged Razorback Suckers. The 300 foot long trammel nets are comprised of an outer layer of 12 inch mesh and an inner layer of 1, 1.5, or 2 inch mesh that captures Razorback Suckers. Larval samples are collected by submerging lights in several inches of water and capturing larval fish with aquarium-type dip nets as they swim toward the lights. 

Lessons Learned

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Surveys for Razorback Suckers in Lake Mead have incorporated the best available techniques since their start in the 1990s. A key takeaway for effectively monitoring a large system like Lake Mead is that partners cannot rely on a single sampling technique. Research crews use everything from trammel nets to remote passive integrated transponder (PIT) tag scanners, and even larval light traps, to increase the likelihood of detecting as many fish as possible. They continue to find new Razorback Suckers during every survey. 

Environmental observations including substrate type, water temperature, water depth, and turbidity allow research crews to locate areas in which larval Razorback Suckers are likely to survive. The three inflows to Lake Mead (the Colorado River, the Virgin River, and the Las Vegas Wash) create centralized areas of high turbidity and slightly warmer temperatures in which larval Razorback Suckers thrive. Systems that do not have muddy inflows leave larval Razorback Suckers exposed to predation and these populations must be supplemented. Given the wide temperature range discussed above, researchers have found that Razorback Suckers actively seek the warmer end of temperatures.

Allocating resources to new monitoring techniques, such as remote PIT tag scanning and eDNA, may significantly improve field results and focus efforts on areas populated with Razorback Suckers.  New schools of Razorback Suckers are sometimes found in little-known areas with similar temperature, water characteristics, and geological conditions to known Razorback Sucker hotspots. These areas can only be further investigated by field crews if additional personnel, financial resources, and time are available to explore new locations.

Research crews are often approached by curious members of the public and are enthusiastic to share their findings and experience working towards the conservation of Razorback Suckers. Many young members of the public are unaware of the species while those of an earlier generation may remember when these fish were more abundant. Conservation efforts rely on these positive memories and, for this reason, it is important to share meaningful information with people in the field as well as to publish technical reports.

Next Steps

• Continue Monitoring: Yearly monitoring of the Lake Mead Razorback Sucker population will continue, to include traditional survey methods as well as improved technology and genetic surveys.
• Research Management Cycle: New research questions about Razorback Suckers from long-term monitoring efforts also inform further experimentation, both in the field and in the lab. Results from this can be used to improve management strategies.
• Public Outreach: Minimal information about the Lake Mead population of Razorback Suckers is common knowledge and there are active efforts to make these research and monitoring activities more widely known to the public including one-on-one interactions with biologists in the field and published literature.

Funding Partners

• Bureau of Reclamation
• Lower Colorado River Multi-Species Conservation Program
• Southern Nevada Water Authority

Resources

• April 2017 National Park Service Lake Mead Water Use
• August 2018 Razorback Sucker Species Status Assessment
• LCR MSCP Razorback Sucker Investigations at the Colorado River Inflow Area Lake Mead, Nevada and Arizona 2010 Final Annual Report
• LCR MSCP Razorback Sucker Studies on Lake Mead, Nevada and Arizona 2010-2011 Final Annual Report
• LCR MSCP Lake Mead Razorback Sucker Monitoring Recommendations, 2005-2006 Report
• BIO-WEST Fisheries and Aquatic Ecology Services 

Contacts

• Jim Stolberg, Fisheries Group Manager, Lower Colorado River Multi-Species Conservation Program, jstolberg@usbr.gov
• Brandon Senger, Supervising Fisheries Biologist, Nevada Department of Wildlife, bsenger@ndow.org
• Jessica Gwinn, Fish and Wildlife Biologist-Colorado River Coordinator, Fish and Wildlife Service, jessica_gwinn@fws.gov

Case Study Lead Author

Caitlyn Aymami, CART Undergraduate Research Assistant, University of Arizona

Suggested Citation

Aymami, C.G. (2020). “Tracking Razorback Sucker Recruitment in Lake Mead.” CART. Retrieved from https://www.fws.gov/project/razorback-suckers-lake-mead.