Part of a series of articles titled Intermountain Park Science: Drought in the Southwest.
Article • Intermountain Park Science: Drought in the Southwest
Smallmouth Bass: Not a Small Threat to the Grand Canyon
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Melissa Trammell, Fisheries Biologist, National Park Service
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Brian Healy, Supervisory Research Fish Biologist, Southwest Biological Science Center, U.S. Geological Survey, Grand Canyon Monitoring and Research Center
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Kirk Young, Biologist, U.S. Fish and Wildlife Service (retired)
Abstract
Decades of climate-driven drought, and a consistent imbalance between supply and demand for Colorado River water have led to low reservoir levels in the Colorado River Basin. Low water levels have implications to rivers located both upstream and downstream of the reservoirs, particularly in the Colorado River in Grand Canyon, located below Glen Canyon Dam, and above Lake Mead. Lower water elevation in Lake Powell brings the warmer surface water and nonnative game fish in closer proximity to the power turbine intakes where they can become entrained and may survive passage to the river below. Smallmouth bass (Micropterus dolomieu), a popular and abundant game fish in Lake Powell, are undesired invasive predators below the dam. Preventing the establishment of a new predatory invasive species is essential to protecting and facilitating recovery of the federally listed humpback chub (Gila cypha) in the Colorado River in Grand Canyon. We modeled the probability that smallmouth bass would establish a self-sustaining population below the dam under a range of scenarios varying lake elevation, release temperatures, survival, and growth. We also evaluated ways to prevent entrainment and to respond to an invasion should it occur. Modeling suggested that altering dam operations to reduce the temperature of water releases to below 16°C for all or a portion of the year could be the most effective treatment to prevent establishment of smallmouth bass below the dam. This approach was implemented in 2024 and was successful in preventing spawning of smallmouth bass below Glen Canyon Dam down to the confluence of the Little Colorado River, 76 miles downstream.
Lake Levels and Warmwater Fishes
Decades of climate-driven drought (Udall and Overpeck 2017, Wahl et al. 2021), and a consistent imbalance between supply and demand for Colorado River water (Wheeler et al. 2021) have led to low reservoir levels in the Colorado River Basin. As the public becomes increasingly aware of the emptying of Lakes Powell and Mead, as the photos and the stories about water shortages, drought, climate change, and warming temperature are increasingly frequent, it is difficult for those who live in the Colorado River Basin to escape the sense of impending disaster. The public may be aware of water shortages affecting cities and farmers, threats to hydropower production, and the impacts to recreation from the loss of almost all boat ramps at Lake Powell, but there are more nuanced changes coming as well.The low lake levels at Powell can result in warm, productive surface waters coming into close proximity of the intakes for the power-generating turbines (Figure 1). Warm-water game fish such as smallmouth bass (Micropterus dolomeiu) primarily inhabit the top 46 feet (14 m) of the reservoir (USBR 2012). As the surface elevation declines, the water that spins the turbines can also carry with it the fish that live in Lake Powell (EPRI 1992, Svoboda 2022). Fish that survive the passage may find suitable habitat in the Colorado River below Lake Powell in Glen Canyon National Recreation Area (NRA) and Grand Canyon National Park (NP; Dibble et al. 2021). The invasion and possibility of establishing a reproducing population of smallmouth bass in the Grand Canyon is of concern, as this species is highly predacious (Figure 2) and implicated in the decline and loss of native fishes throughout the West including the loss of a humpback chub population in Dinosaur National Monument (Bestgen 2016, Finney 2006, USFWS 2018).
Partners in the Glen Canyon Dam Adaptive Management Program became aware of this threat in 2021, and responded by forming the Smallmouth Bass Task Force and asking for an assessment report. The task force was led by the U.S. Fish and Wildlife Service (USFWS) to assess the risk of the smallmouth bass establishing below the dam and begin developing responses to the expected invasion.
The Colorado River and its tributaries in Grand Canyon are home to the largest remaining populations of humpback chub, which was recently down-listed from federally endangered to threatened (USFWS 2021) based, in part, on the status of the species in Grand Canyon. Populations of other native fishes of conservation concern also use this segment of the basin, including the endemic razorback sucker (Xyrauchen texanus) that moves between reservoirs and the rivers that drain into them, provided barriers are not present (Van Haverbeke et al. 2017). In the last decade, the humpback chub has thrived in the Grand Canyon. Numbers have rebounded from previous lows, and they have expanded downstream from their population center near the Little Colorado River inflow. The fish community in the Colorado River in Grand Canyon is now largely native, with humpback chub representing a substantial portion of that community (Van Haverbeke et al. 2017).
Unlike most other reaches of the Colorado River (Martinez et al. 2011), nonnative fish are relatively uncommon throughout the Grand Canyon and smallmouth bass are rare. However, the changes in river temperature that are coming—that are here now—are expected to make the river much more suitable for smallmouth bass growth and reproduction. The increased likelihood of smallmouth bass passing through the dam and surviving will make it easier for the species to overcome the Allee effect (Stephens and Sutherland 1999)— a density limitation where the more individuals are present in a system, the more likely they are to find a suitable mate and reproduce. If smallmouth bass establish a reproducing population, the species can be expected to have a negative impact on the native fish community in the Grand Canyon (Loppnow et al. 2013).
Previous studies suggested that most fish in Lake Powell inhabit the uppermost (14 m) of the lake, and fish passage through the dam would become more likely as the surface elevation approached 3,516 feet, or approximately 46 feet (14 m) above the intakes (USBR 2012). However, fish passage was documented in the spring of 2022 when the lake reached about 3,525 feet, as gizzard and threadfin shad (Dorosoma cepedianum, D. petenense) were observed below the dam, both alive and dead, and green sunfish (Lepomis cyanellus) and bluegill (Lepomis macrochirus) were captured in higher than usual numbers (NPS unpublished data). Warmwater fish surviving passage through the dam was no longer theoretical, but only too real.
The Smallmouth Bass Task Force members and the authors of this paper include agency fish biologists, hydrologists, and ecologists. As subject matter experts, they are familiar with the species, the area, and the laws and policies governing the management of water, power, and natural resources of Glen Canyon NRA and Grand Canyon NP. The Task Force met to compile relevant literature and model the risk and potential outcomes of invasion and control of smallmouth bass, before presenting to the Glen Canyon Dam Adaptive Management Program Technical Work Group. Four subgroups were formed, each led by a member of the task force to take on part of the assessment.
Unlike most other reaches of the Colorado River (Martinez et al. 2011), nonnative fish are relatively uncommon throughout the Grand Canyon and smallmouth bass are rare. However, the changes in river temperature that are coming—that are here now—are expected to make the river much more suitable for smallmouth bass growth and reproduction. The increased likelihood of smallmouth bass passing through the dam and surviving will make it easier for the species to overcome the Allee effect (Stephens and Sutherland 1999)— a density limitation where the more individuals are present in a system, the more likely they are to find a suitable mate and reproduce. If smallmouth bass establish a reproducing population, the species can be expected to have a negative impact on the native fish community in the Grand Canyon (Loppnow et al. 2013).
Previous studies suggested that most fish in Lake Powell inhabit the uppermost (14 m) of the lake, and fish passage through the dam would become more likely as the surface elevation approached 3,516 feet, or approximately 46 feet (14 m) above the intakes (USBR 2012). However, fish passage was documented in the spring of 2022 when the lake reached about 3,525 feet, as gizzard and threadfin shad (Dorosoma cepedianum, D. petenense) were observed below the dam, both alive and dead, and green sunfish (Lepomis cyanellus) and bluegill (Lepomis macrochirus) were captured in higher than usual numbers (NPS unpublished data). Warmwater fish surviving passage through the dam was no longer theoretical, but only too real.
The Smallmouth Bass Task Force members and the authors of this paper include agency fish biologists, hydrologists, and ecologists. As subject matter experts, they are familiar with the species, the area, and the laws and policies governing the management of water, power, and natural resources of Glen Canyon NRA and Grand Canyon NP. The Task Force met to compile relevant literature and model the risk and potential outcomes of invasion and control of smallmouth bass, before presenting to the Glen Canyon Dam Adaptive Management Program Technical Work Group. Four subgroups were formed, each led by a member of the task force to take on part of the assessment.
The Smallmouth Bass Task Force was formed at the beginning of February 2022 and presented an analysis and summary presentation at their scheduled April 2022 virtual meeting. They were able to act so quickly in large part due to the critical ecological modeling completed by Charles Yackulic and Drew Eppehimer of the U.S. Geological Survey-based Glen Canyon Monitoring and Research Center.
The Smallmouth Bass Task Force began with the short-term analysis of the effects of water quality, particularly temperature changes, on the fish community below the dam. We identified data gaps and gathered information from the literature and a concurrent study of fish community in the forebay area—a study conducted by Utah State University (Barrett Freisen 2022). The data were used to quantify fish densities, depth distribution, and movement in Lake Powell in close proximity to the dam. The analyses and other considerations were intended to assess the risk of smallmouth bass establishing a reproducing population below Glen Canyon Dam and initiate the potential short-term and long-term responses.
The four task force subgroups focused on temperature, entrainment, limitations, and other management alternatives. The temperature subgroup evaluated how the expected release water temperature would affect smallmouth bass growth and reproduction, and how alternative release operations may affect temperature and smallmouth bass establishment in downstream reaches. The entrainment group evaluated the risk of entraining fish through the penstocks at a range of reservoir elevations and estimated the abundance of fish available to be entrained in the reservoir forebay area, as well as potential survival rates of passage through the dam, and subsequent annual survival. The limitations group focused on physical and biological factors that might limit the establishment of smallmouth bass below the dam. The group looking at other management alternatives assessed the potential for preventing entrainment and short-term responses to invasion. The assessment results were organized into a presentation outline that laid out a logical progression from defining the problem and assessing the risk, to evaluating potential responses.
The Smallmouth Bass Task Force began with the short-term analysis of the effects of water quality, particularly temperature changes, on the fish community below the dam. We identified data gaps and gathered information from the literature and a concurrent study of fish community in the forebay area—a study conducted by Utah State University (Barrett Freisen 2022). The data were used to quantify fish densities, depth distribution, and movement in Lake Powell in close proximity to the dam. The analyses and other considerations were intended to assess the risk of smallmouth bass establishing a reproducing population below Glen Canyon Dam and initiate the potential short-term and long-term responses.
The four task force subgroups focused on temperature, entrainment, limitations, and other management alternatives. The temperature subgroup evaluated how the expected release water temperature would affect smallmouth bass growth and reproduction, and how alternative release operations may affect temperature and smallmouth bass establishment in downstream reaches. The entrainment group evaluated the risk of entraining fish through the penstocks at a range of reservoir elevations and estimated the abundance of fish available to be entrained in the reservoir forebay area, as well as potential survival rates of passage through the dam, and subsequent annual survival. The limitations group focused on physical and biological factors that might limit the establishment of smallmouth bass below the dam. The group looking at other management alternatives assessed the potential for preventing entrainment and short-term responses to invasion. The assessment results were organized into a presentation outline that laid out a logical progression from defining the problem and assessing the risk, to evaluating potential responses.
A Summary of the Assessment Results
Why Worry About Smallmouth Bass?
Smallmouth bass have been implicated in the decline of native fish throughout the West (Staudinger et al. 2021). The species is considered the highest predatory threat in the Colorado River Basin (Johnson and Martinez 2008) and one of the four highest-risk species in the National Park Service Expanded Non-native Aquatic Species Environmental Assessment (NPS 2019). Dramatic changes to the Grand Canyon’s native fish community could occur if invasive warmwater predators such as smallmouth bass become established.
Why are Smallmouth Bass Not Already Here?
Until recent low water levels in Lake Powell, water temperatures in the Colorado River below Glen Canyon Dam were unsuitable for warmwater invasive species as dam release temperatures seldom exceeded 57°F (14°C), below the minimum spawning temperature for smallmouth bass of 60.8°F (16°C; Figure 3). Because Lake Powell reservoir elevation was higher than 3,525 feet, few fish were entrained through the penstocks. Although the river warms during the summer as it flows downstream, few smallmouth bass have been present in the system to respond to warming temperatures (Figure 4). There are smallmouth bass present in Lake Mead, downstream of the Grand Canyon; however, few have been captured in the downstream end of the Grand Canyon. Pearce Ferry rapid, formed in 2007 at the receding inflow to Lake Mead, appears to act as a partial, perhaps nearly complete, barrier to upstream fish movement for both the warmwater sportfish and the endangered razorback sucker in Lake Mead, likely because of high velocity and turbulence (Van Haverbeke 2017).
U.S. Geological Survey
Spatial distribution of effort differed over time. Data and map provided by the U.S. Geological Survey.
What is Changing?
Declining reservoir elevation at Lake Powell led to increased entrainment potential, and warmer water releases, making the Colorado River in Grand Canyon more suitable for smallmouth bass spawning, growth, and survival.
Risk Assessment
How Reversible is a Smallmouth Bass Invasion?
Four scenarios of increasing levels of risk of invasion were described. There is more potential to reverse an invasion at the early stages of establishment, but invasion science strongly suggests that prevention of invasion is preferred to attempts to reverse an invasion, as reversal is seldom successful (Ricciardi et al. 2020). Scenario 1 considered smallmouth bass numbers increasing in Lees Ferry or downriver, but not reproducing. Invasion science suggests that invasions are easier to reverse if new invaders are prevented or removed before reproduction can occur. Scenario 2 advances to an established population of smallmouth bass in Lees Ferry, with reproduction occurring in most years. We considered this situation perhaps reversible over many years if reproduction is successfully disrupted every year. Scenario 3 would occur when there are one or more established populations downriver from Lees Ferry in the mainstem river. This situation is only reversible with extreme changes to dam operations required on a decadal time scale. Scenario 4 is reached when populations become established in tributaries, which cannot be affected by dam operations. This situation is unlikely to be reversible even with extensive removal efforts as well as extreme changes to dam operations. Because all scenarios are difficult or impossible to reverse, prevention of entrainment is seen as the best solution, but that also is difficult, expensive, and requires considerable planning and funding.
How Much Entrainment is Required to Establish a Smallmouth Bass Population?
We estimated the number of smallmouth bass likely needed to establish a population to be between 9 and 46 adults per km (Martinez et al. 2014; i.e., the number needed to overcome the Allee effect). With that as a potential target, we calculated the number of smallmouth bass likely to be entrained using literature-derived values for estimated annual survival, survival of passage through the penstock turbines, entrainment rate based on the area of the intake zone, and abundance of smallmouth bass in Lake Powell in the area near the intake zone. We assumed a range of values for an estimated number of smallmouth bass present in Lees Ferry at the beginning and ending of a year (Figure 5). We compared the modeled number of smallmouth bass present at the end of a year to the estimated number of spawners needed to establish a population in Lees Ferry to produce a range of probabilities of establishment. Since fish can accumulate over one or more years, the probability increases with additional years (Figure 6).
Are Conditions Suitable?
Conditions were expected to be suitable in the Colorado River below Glen Canyon Dam from the dam downriver to Lake Mead in 2022 based on minimum temperature needed for smallmouth bass reproduction and growth (16°C or above) occurring from June – September, throughout the summer reproduction period (Figure 7). In addition, zero- or very low-velocity habitat is needed for successful reproduction. While uncommon in the Grand Canyon, low-velocity habitat does occur and could support nesting and reproduction. Conditions are also suitable in several tributaries, from the Little Colorado River to Kanab Creek and downstream (see Figure 4 for locations of tributaries).
U.S. Bureau of Reclamation
Next Steps
What Can Scientists Do?
Scientists can continue to monitor smallmouth bass and other warmwater invasive fish species below Glen Canyon Dam and continue to refine the risk assessment. With these data, we can model the efficacy of various prevention, removal, and control strategies to provide decision makers with the best strategies to prevent or respond to the problem. The DOI (2021) Invasive Species Strategic Plan (2021) suggests that:
“…for invasive species that fail to be excluded through prevention efforts, early detection and rapid response…can avoid the long-term costs and economic burden that invasive species might otherwise cause.”
What Can Decision Makers Do?
Decision makers can design and implement science-based actions to prevent and respond to a potential irreversible invasion. Limiting entrainment and controlling release temperatures are the only large-scale tools currently available. Timely response to a small number of smallmouth bass may be successful if response time is rapid enough to control reproduction and spread to prevent establishment.
Discussion
Many factors affect the likelihood of entraining fish through the penstocks, including the elevation of the reservoir surface relative to the penstocks, the depth at which fish species occur, the abundance of the fish, and the volume of water passing through the dam. Each of these are difficult to quantify, but certain relationships can be defined. Risk of entrainment increases as the reservoir elevation approaches the penstock level, rising quickly as surface elevation is within 50 feet of the penstocks, and then drops off again as the reservoir falls below minimum power pool at 3,490 feet, below which hydropower production ceases and water must be released through the bypass tubes (Figure 6). This risk can be theoretically altered by adding volumes of water to the reservoir to raise the elevation and lower the release temperatures. While this concept may seem unlikely, an existing 2019 Drought Response Operations Agreement between the Upper and Lower Colorado River basin states transferred an additional half-million acre feet of water into Lake Powell in 2022 from upstream reservoirs, primarily Flaming Gorge Reservoir. Another half-million acre feet were retained in Lake Powell and not delivered to the lower basin (DROA 2022). These agreements were made with the intention of keeping Lake Powell above the minimum power pool so that hydropower could still be generated, but the effect may reduce entrainment risk as well.
The risk of entrainment could be further reduced by keeping the reservoir elevation higher, releasing cooler water through the bypass tubes, or by constructing a screening device in Lake Powell or just below the dam. The U.S. Bureau of Reclamation and the Upper and Lower Basin States are continuing to work toward maintaining Lake Powell water elevation above power pool, and the Bureau is evaluating the efficacy of using the bypass tubes to cool the release temperatures and the resulting impact on hydropower production. Svoboda (2022) evaluated solutions including physical screens and behavior modification devices such as bubble screens that guide fish away from the intakes. Each of these solutions would take considerable time and funding to implement. Although preventing entrainment is preferred to in-river removal and control, some in-river management must occur in the interim. Preventing establishment will require rapid response to any observed smallmouth bass in the river to remove them before they reproduce.
If entrainment cannot be prevented, the risk of establishment increases over time, as fish surviving passage through the dam can accumulate in Lees Ferry. As fish accumulate and approach or exceed the Allee threshold, other habitat suitability factors come into play that affect reproductive success. Also necessary for successful reproduction are sufficient food, low turbidity, suitable spawning habitat with low velocity, gravel or cobble substrate available at a range of flows, and suitable temperature. An abundant supply of nonnative rainbow trout (Onchorhynchus mykiss) occurs in Lees Ferry that could provide become prey for smallmouth bass. Although water released from Glen Canyon Dam is clear, occasional turbidity from tributary sediment could limit smallmouth bass reproduction in Lees Ferry and downstream. Our analysis of availability of spawning habitat revealed 49-124 acres (20-50 ha) of suitable habitat was available under normal discharges ranging from 5,000 to 45,000 cfs (141.6 to 1274.2 cms), which is sufficient to support a spawning population of smallmouth bass. These factors are difficult to manage or alter.
Spawning temperature could limit smallmouth bass and is likely to be the primary driver of reproductive success. Dam release temperatures can be affected by reservoir elevation, as well as whether water is released through the penstocks (currently warm and suitable), or through the bypass tubes, another 100 feet (30 m) or so below the penstocks (cool and unsuitable). Modeling suggested that altering dam operations to reduce the temperature of water releases to below 60°F (16°C) for all or a portion of the year could be the most effective treatment to prevent establishment of a new population of smallmouth bass below the dam. However, releasing water through the bypass tubes also affects the production of hydropower. Additional National Environmental Protection Act compliance was necessary to reoperate the dam to achieve cooler temperatures. The Bureau of Reclamation began working on the environmental compliance processes to evaluate alternatives relative to implementing this concept in 2022.
The situation continued to evolve. After the Smallmouth Bass Task Force conducted the risk assessment in 2022, partner agencies responded by increasing the frequency and area of monitoring below Glen Canyon Dam with additional surveys conducted by the National Park Service and other task force partners. A citizen science informational flyer about smallmouth bass was created to engage river runners in looking for smallmouth bass in clear tributaries. River temperatures continued to rise and the river in Lees Ferry was above 60°F (16°C) from mid-June through October 2022. Several adult smallmouth bass were captured or observed below the dam, and 20 smallmouth bass fingerlings were captured in a large backwater about 3 miles (4.8 km) below the dam, suggesting local reproduction had occurred. All captured fish were removed, and a piscicide treatment was completed in September of 2022 in the relatively isolated backwater as a rapid response to this realized invasion. After the piscicide treatment (NPS unpublished data), monitoring captured 37 smallmouth bass fingerlings in the Lees Ferry reach outside the treatment area (GCMRC unpublished data). These fish may have been produced locally or may have passed through the dam. Additional monitoring continued to observe smallmouth bass in the Lees Ferry Reach.
A study was designed and implemented to evaluate if repeated electrofishing to remove smallmouth bass in Lees Ferry could have a depletion effect (USGS 2022). Six removal trips were completed from November through December 2022. Capture probability was calculated for independent 2-mile (3-km) segments after multiple passes were completed. Capture probabilities were variable between 1% and 60%, and a depletion effect was only seen in the segment closest to the dam, which also had the highest abundance of smallmouth bass. Only two adults were captured, with the remaining fish between 2 and 4 inches (60 to 100 mm) total length, presumed to be Age 0 (Tennant et al. 2023). Although 320 smallmouth bass were removed, the capture probabilities suggest that many fish remained; thus, more removal efforts were needed to continue to address this problem while long-term solutions continue to be sought and implemented.
The winter of 2022-2023 saw better-than-average snowfall across the Upper Colorado River basin. From a low of below 3,520 feet NGVD 29 in April 2023 (Figure 8), runoff from the melting snowpack increased the Lake Powell reservoir level by early July to 3,582 feet, more than 90 feet above the power plant intakes. Reservoir annual release volume was reduced to 7.48 million acre feet in 2023 and 2024, according to the 2007 Interim Guidelines for Colorado River Operations (USBR 2007) and other existing agreements, so the reservoir remained high through 2023 and into 2024. Entrainment of smallmouth bass likely continued in the early part of 2023 before runoff began, but little entrainment was observed in 2024, although some entrainment of green sunfish was observed, and we assumed some smallmouth bass were also coming through the dam. In 2023, despite the increase in reservoir elevations, release temperatures remained elevated through the summer and into fall, allowing reproduction to occur in the mainstem as well as in the slough (Figure 9). A block net was placed in the slough to reduce reinvasion and prevent reproduction in the slough, but it was not completely successful. Reproduction of smallmouth bass was again seen in the slough, and another piscicide treatment in the slough was implemented in August of 2023. However, smallmouth bass continued to be captured in the mainstem, as monitoring and removal efforts continued through November. Summer and fall of 2023 saw the highest overall captures of smallmouth bass below the dam, including fish removed by the piscicide treatment. Thus, the partners all recognized that the suppression and control efforts must continue—and expand.
U.S. Geological Survey
The U.S. Bureau of Reclamation pursued the concept of a release of colder water through the dam, bypassing the hydroelectric plant which culminated in an Environmental Impact Statement (EIS; USBR 2024). The EIS evaluated several options for cooling the water and chose to implement the “cool-mix” alternative in the summer of 2024. Releases through the hydroelectric power plant were reduced, but continued, and were mixed with carefully calculated volumes of cold (average of 50oF [10oC]) releases through the bypass tubes located 120 feet (36 m) lower than the power intakes. Combined releases targeted temperatures of 59.9oF (15.5oC) or less at River Mile 61 downstream, near the confluence of the Little Colorado River, an important spawning site for humpback chub. The intent was to keep the daily average water temperature below critical spawning temperatures for smallmouth bass, preventing reproduction in the first 76 miles of river below the dam. Cool-mix was implemented beginning July 9, 2024, just as river temperatures began to rise above the critical temperature. The mixed releases continued through the early November, until temperatures naturally fell below 59.9oF (15.5oC) at Lees Ferry, 15 miles below the dam (Figure 9). This collaborative effort was successful, and no reproduction was observed in the mainstem of the river in 2024. Concurrent electrofishing efforts continued to remove as many smallmouth bass as possible, before previous-year classes matured enough to begin spawning if temperatures are warmer in future years. Implementation of the cool-mix negatively impacted the production and value of hydropower at Glen Canyon Dam and future implementation will try to further minimize these impacts.
Additional actions were taken to prevent smallmouth bass from entering the slough area and encountering warmer temperatures there. An improved block net was placed across the mouth of the slough and inspected and maintained weekly. Trap nets (fyke and hoop nets) were set inside the slough and checked weekly to assess effectiveness of the block net. No smallmouth bass were captured in the slough inside the block net, so this effort was also successful at preventing reproduction, and no piscicide treatment was necessary in 2024 (NPS unpublished data).
The NPS is pursuing a long-term reduction in suitable spawning habitat by modifying the sloughs. An Environmental Assessment is being prepared, evaluating the impacts of channelizing the slough to increase water velocity and reduce temperature throughout in hopes of preventing spawning in the future. The slough and surrounding cobble bar were low-elevation features when the dam was built, and the slough area was most frequently a flowing side channel with little or no vegetation. The modifications will restore this area to a flowing side channel.
Although smallmouth bass are the focus of this article and the removal efforts, also concerning are green sunfish, which were more abundant than smallmouth bass in the removal efforts and are considered high risk invasive species (NPS 2019). Walleye (Sander vitreus), while still uncommon, have been increasing in numbers and some reproduction has occurred below the dam, and are considered a “very high-risk” invasive species (NPS 2019 Appendix F update 2021). The cool-mix temperatures are more suitable for walleye; thus, this species is likely the next focal species for non-native control efforts.
Additional actions were taken to prevent smallmouth bass from entering the slough area and encountering warmer temperatures there. An improved block net was placed across the mouth of the slough and inspected and maintained weekly. Trap nets (fyke and hoop nets) were set inside the slough and checked weekly to assess effectiveness of the block net. No smallmouth bass were captured in the slough inside the block net, so this effort was also successful at preventing reproduction, and no piscicide treatment was necessary in 2024 (NPS unpublished data).
The NPS is pursuing a long-term reduction in suitable spawning habitat by modifying the sloughs. An Environmental Assessment is being prepared, evaluating the impacts of channelizing the slough to increase water velocity and reduce temperature throughout in hopes of preventing spawning in the future. The slough and surrounding cobble bar were low-elevation features when the dam was built, and the slough area was most frequently a flowing side channel with little or no vegetation. The modifications will restore this area to a flowing side channel.
Although smallmouth bass are the focus of this article and the removal efforts, also concerning are green sunfish, which were more abundant than smallmouth bass in the removal efforts and are considered high risk invasive species (NPS 2019). Walleye (Sander vitreus), while still uncommon, have been increasing in numbers and some reproduction has occurred below the dam, and are considered a “very high-risk” invasive species (NPS 2019 Appendix F update 2021). The cool-mix temperatures are more suitable for walleye; thus, this species is likely the next focal species for non-native control efforts.
Summary
The Smallmouth Bass Task Force was successful in rapidly assimilating information and assessing the likelihood (risk) of the species passing through the dam in sufficient numbers and finding suitable conditions downstream to be able to establish a reproducing population. The risk was found to be high and increased with decreasing reservoir elevation and multiple years of entrainment. Later observations of smallmouth bass passing through and reproducing below Glen Canyon Dam proved the initial risk assessment to be accurate. We recommended a range of actions that could be taken to prevent smallmouth bass establishment if actions could be taken swiftly. The recommendations for additional monitoring and removal were implemented by management agencies, and additional compliance for longer term solutions using flow, habitat modification, and exclusion devices were evaluated, implemented, and are still being explored. While smallmouth bass are still present below Lake Powell, ongoing actions to monitor and control this species will continue to be needed. If reproduction can be suppressed by keeping river temperatures below critical thresholds, the population should decline to a point where much less removal effort is needed to keep the population from becoming established. The Smallmouth Bass Task Force and the ongoing rapid response group efforts to manage the invasive species are an excellent example of a successful collaboration with a dedicated group pooling data and skills across disciplines and agency lines to achieve a useful result.References
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Last updated: January 17, 2025