Difference between revisions of "Smallmouth Bass Page"

As anticipated, the 2024 Cool Mix flows negatively impacted smallmouth bass in Glen Canyon and Grand Canyon, leading to reduced growth, a decline in relative abundance, and no confirmed recruitment or spawning observed. (1)

In 2023, the mean daily catch per unit effort (CPUE) of smallmouth bass between GCD and Lees Ferry increased from May to July. The highest CPUE in 2023 occurred in October when young of the year smallmouth bass grew to catchable sizes. In 2024, SMB relative abundance remained stable throughout the year, indicating that no new fish were growing to catchable size and suggesting that no reproduction occurred. Both juvenile and large adults (i.e., >200 mm), were more abundant in 2023 compared to 2024. Due to low capture rates in 2023-2024, relative abundance estimates for smallmouth bass below Lees Ferry were not calculated.

Length-frequency data revealed the presence of a new year class of smallmouth bass in 2023, shown by the larger proportion and abundance of juvenile smallmouth bass (20–30 mm) in June-August and growing larger into the fall. In 2024, we did not see fish within this size class (20-30 mm) supporting a lack of bass reproduction below the dam in 2024. (2)

Most smallmouth bass continue to be collected in the first couple of miles below Glen Canyon Dam.

From 2022-2024, the furthest downstream smallmouth bass were captured was at RM 16.38R in November 2023. The furthest downstream adult smallmouth bass (>200mm) was captured within the slough at RM -12.19L in July 2023. Distribution of smallmouth bass in 2024 was similar to that observed in 2023 with the exception that bass were largely absent from the -12 mi slough. A total of 252 smallmouth bass were captured across all projects, with 191 of those fish captured in the Glen Canyon reach (RM –15.71 to RM 0). Bass captures in the PBR reach (RM 0 to RM 8) increased slightly in 2024 (13 individuals captured vs 8 in 2023), suggesting that large-scale migration downstream is not yet occurring. (3)

In 2024, environmental DNA (eDNA) samples from both sides of the river every mile from RM 0 - RM 20 and every 5 miles from RM 25 – RM 65. This spans the distance from Lees Ferry to the Little Colorado River. Positive eDNA detections were made down to RM 37.5. In large rivers, eDNA can be detected more than 100 km downstream though the strongest signals are typically within 10–100 meters of the source [2].

Maintaining Lake Powell elevations above 3590 ft would likely minimize entrainment and create downstream conditions that minimize smallmouth bass population growth. Entrainment increases dramatically as reservoir elevation declines below 3530 ft (see the Entrainment page). [3]

There appears to be two cohorts of juvenile bass being collected in the Glen Canyon Dam tailwater. One from 2022 and another from 2023 (2025 Annual Reporting Meeting). Young of the year smallmouth bass were collected from the -12 Mile Slough in both 2022 and 2023 indicating that local reproduction occurred in the slough in those two years. The catch distribution of smallmouth bass in the tailwater being greater nearest the dam indicates either entrainment and/or additional spawning right below the dam may have been also contributing. https://www.usbr.gov/uc/progact/amp/twg/2025-04-10-twg-meeting/uc-gcdamp-twg-arm-smallmouthbass-coolmix-508-20250410.pdf

Release temperatures from Glen Canyon Dam suggesting that epilimnetic releases were made in 2022 (yellow) and 2023 (purple), but not in 2021 (orange) or 2024 (green). Release temperatures for 2024 are presented here without mixing the cooler bypass releases. A higher likelihood of entrainment is associated with epilimnetic releases like those that occurred in 2022 and 2023.

Smallmouth bass have been sporadically captured in Grand Canyon since 2003. Captures continue to be concentrated below Glen Canyon Dam but individuals have been caught in the past at the Little Colorado River confluence and in western Grand Canyon near the Lake Mead inflow. They were first introduced into Lake Powell in 1982 [4] and there are populations in ponds and lakes in the upper Little Colorado River and were first detected in Lake Mead in 2000 [5].

Pine et al. 2025. Assessing the potential for Smallmouth Bass population establishment in Grand Canyon

The expansion of nonnative Smallmouth Bass (Micropterus dolomieu) into the Grand Canyon ecosystem downstream of Glen Canyon Dam is widely seen as a potential threat to native fishes, particularly the Humpback Chub (Gila cypha), which is listed as threatened under the Endangered Species Act. This concern stems from observations in the Green and upper Colorado rivers, where dam- modified habitats and reservoir introductions have allowed Smallmouth Bass to become established and impact native fish species. Our objective was to compare habitat conditions between a Smallmouth Bass population center in the Green River— where Humpback Chub are rare— and two major Humpback Chub population centers in the Colorado River in Grand Canyon, where Smallmouth Bass invasion is a conservation concern, to help assess invasion risk and inform management actions downstream of Glen Canyon Dam.

Methods

We developed a conceptual model of Smallmouth Bass recruitment potential emphasizing the roles of temperature, turbidity, and the timing of these conditions, based on literature for Smallmouth Bass and Largemouth Bass Micropterus nigricans. Using U.S. Geological Survey gauge station data, we summarized daily temperature and turbidity patterns near a known Smallmouth Bass population center in the middle Green River (near Jensen, Utah) and compared those with conditions in the Colorado River in Glen Canyon and Grand Canyon downstream from Glen Canyon Dam. We then used our conceptual model of Smallmouth Bass early life history, along with observed turbidity and temperature patterns, to explore how irregular recruitment could affect Smallmouth Bass population dynamics and individual growth potential using an age- structured population model and a bioenergetics model.

Results

Although warm reservoir releases due to low water storage in Lake Powell has made temperature regimes in Glen and Grand canyons more similar to those of the middle Green River, the Colorado River ecosystem in Grand Canyon has a very different turbidity regime from that of the middle Green River. The Colorado River in Grand Canyon becomes very turbid during late summer and fall, when tributaries experience flash floods during the North American monsoon season. These periods of high turbidity coincide with critical early life stages of Smallmouth Bass, likely limiting foraging efficiency, growth, and overwinter survival. Our population model indicates that rapidly growing and sustained Smallmouth Bass populations require successful recruitment at least once every 2 years, but empirical data suggest that these conditions occur only every 5–7 years in Grand Canyon. This would make long- term establishment of self- sustaining Smallmouth Bass populations in Grand Canyon unlikely or at least highly uncertain.

Conclusions

Despite the presence of nonnative predators, Humpback Chub populations in Grand Canyon have expanded during the past 10–15 years, very likely due to favorably warm river temperature. While Smallmouth Bass in Glen or Marble canyons (between Glen Canyon Dam and the Little Colorado River) may pose a downstream dispersal risk (and potentially a threat to Humpback Chub populations), high turbidity conditions downstream of the Little Colorado River reduce the likelihood of persistent local Smallmouth Bass recruitment. These results suggest that Smallmouth Bass invasion risk to Humpback Chub is not uniform across the Colorado River basin and should be evaluated in the context of the different site- specific water quality and flow regime conditions, other known risks to Humpback Chub, and Humpback Chub population size in different parts of the basin.

• SEIS Bypass Releases Page
• The -12 Mile Slough Page
• Turbidity Page
• Entrainment Page
• Nonnative Invasive Aquatic Species Page
• 2023 Invasive Fish Species Below Glen Canyon Dam: A Strategic Plan to Prevent, Detect and Respond
• 2019 National Park Service Expanded Non-native Aquatic Species Management Plan
• 2013 National Park Service Comprehensive Fisheries Management Plan
• Habitat suitability information: Smallmouth bass

• SEIS Bypass Releases at Glen Canyon Dam
• Evaluate effects of flow spikes to disrupt reproduction of smallmouth bass in the Green River downstream of Flaming Gorge Dam

2026

• Pine et al. 2025. Assessing the potential for Smallmouth Bass population establishment in Grand Canyon. Transactions of the American Fisheries Society
• Supplemental Information for Pine et al. 2025. Assessing the potential for Smallmouth Bass population establishment in Grand Canyon. Transactions of the American Fisheries Society
• Yackulic et al. 2026. Comment on “Assessing the potential for Smallmouth Bass population establishment in Grand Canyon.” Transactions of the American Fisheries Society
• Pine et al. 2026. Response to Comment on “Assessing the potential for Smallmouth Bass population establishment in Grand Canyon.” Transactions of the American Fisheries Society
• Smallmouth Bass and Nonnative Fish Monitoring in 2025

2025

• Eppehimer et al., 2025, Declining reservoir elevations following a two-decade drought increase water temperatures and non-native fish passage facilitating a downstream invasion: Canadian Journal of Fisheries and Aquatic Sciences
• SBAHG Update
• Smallmouth Bass and Other High-Risk Nonnatives
• Smallmouth Bass and other Nonnatives
• Smallmouth Bass Updates
• SBAHG Update
• Tucker Trawl and Hydroacoustic Update
• Smallmouth Bass: Not a Small Threat to the Grand Canyon
• Response of Smallmouth Bass and other species to cool mix

2024

• Friesen, Barrett T., "Invasion Potential of Nonnative Fishes Through a Large Western Dam Into a Prized and Vulnerable Ecosystem" (2024). All Graduate Theses and Dissertations, Fall 2023 to Present. 288. https://digitalcommons.usu.edu/etd2023/288
• 2024 Cool Mix Flow Summary Report
• Review of Smallmouth Bass Management in the Colorado River Ecosystem
• SBAHG Update
• Smallmouth Bass Updates
• 2024 Bass Nest Monitoring
• Smallmouth Bass Updates
• Fisheries Review: Annual Reporting FY2023
• Smallmouth Bass Population Modeling and Implications

2023

• SBAHG Update & Nonnative Fish Strategic Plan: Review and Discussion
• Smallmouth Bass Depletion Effort in Glen Canyon National Recreation Area

2022

• Smallmouth Bass Ad Hoc Group: Review, updates, and next steps
• Near-Term Threat of Smallmouth Bass Establishment below Glen Canyon Dam
• 'Worst fears confirmed' in biologists' fight to save ancient Colorado River fish
• Flow spike to reduce smallmouth bass reproductive success, Green River, 2021
• Near-Term Threat of Smallmouth Bass Establishment below Glen Canyon Dam
• Fish modelling to support management decisions

2016

• Bestgen and Hill. 2016. River regulation affects reproduction, early growth, and suppression strategies for invasive smallmouth bass in the upper Colorado River basin.

Description: Smallmouth bass are a non­native fish that was introduced into Arizona in 1921. They are native to the upper Mississippi River basin. These bass are most often bronze to brownish in color, with dark vertical bars on the sides. In contrast to the largemouth bass, the upper jaw does not extend beyond the rear margin of the eye. The eye is reddish in color and there is a shallow notch in the dorsal fin. The soft dorsal fin has 13 to 15 rays. Length can vary between 12 and 22 inches, and smallmouth bass can weigh between 8 ounces and 7 pounds. [6]

Location and Habitat: Within its native range the smallmouth bass seems most abundant in pools of streams that consist of a substantial proportion of riffle habitat, clean, rocky, hard bottoms, and gradients of 0.5 to about 5.0 m per km. In large rivers and lakes, smallmouth bass tend to congregate over hard, stony bottoms, where currents are present. At the present time, smallmouth bass occur in the mainstream of the Colorado River, in the Verde River system, and throughout the Salt River Basin below about 2,200 meters in elevation. [7]

Temperature: Temperatures may be the most important single factor limiting distribution of smallmouth bass. Faster growth rates of adult smallmouth bass are generally associated with higher summer temperatures. Faster growth rates occur in southern reservoirs, resulting in earlier death than in northern regions. In the summer, bass inhabit warmer shoreline areas of large lakes in the North and deeper, cooler waters in the South. Growth does not begin until water temperatures reach 10-14° C. Field data indicate that adults prefer temperatures of about 21-27° C in the summer. Smallmouth bass have been reported "sunning" themselves in pools with water temperatures of about 26.7° C in summer. [8]

Temperature preferences of smallmouth bass vary considerably depending on the acclimation temperature. Smallmouth bass acclimated at 2.2-30.0° C selected temperatures of 20-32° C in laboratory tests. Adult bass in the laboratory preferred temperatures of 28° C to 31° C. Optimum growth rates in the lab occurred at temperatures from 26-29° C. Upper lethal temperatures for adults were above 32.3° C. [9]

When temperatures drop to 15-20° C, adults seek deep, dark areas. At about 10° C, bass become inactive and seek shelter. At 6-7° C, most smallmouth bass are beneath the rock substrate, with few remaining on top. The lower lethal temperature is near freezing. Bass will congregate around warm springs in winter when available.[10]

Turbidity and Dissolved Oxygen: Smallmouth bass apparently can tolerate periodic turbidity, although excessive turbidity and siltation will reduce a population. Hubert and Lackey (1980) reported a typical smallmouth bass habitat to have very low turbidity, usually < 25 JTU, and almost never > 75 JTU (except under flood conditions when turbidity is sometimes as high as 250 JTU).[11]

Salinity: Smallmouth occur at pH levels of 5.7 to 9, although optimum pH is 7.9-8.1. Butler (1972) found that smallmouth bass cover-seeking behavior was reduced at pH levels < 6, and the lower lethal pH level was 3. Smallmouth bass populations are more productive in clearer, less fertile reservoirs several years after impoundment that have low total dissolved solids (TDS = 100-350 ppm). [12]

Reproduction: Smallmouth bass spawn in spring, usually mid-April to July, depending on geographical location and water temperature. Cleary (1956) observed a 45 day post-nesting period for smallmouth spawning in Iowa streams. Smallmouth bass spawn on rocky lake shoals, river shallows, or backwaters or move into creeks or tributaries to- spawn. The species requires a clean stone, rock, or gravel substrate for spawning. Studies show that the habitat condition during spawning is the most important factor for year class strength in smallmouth bass. Nest building and spawning occur when the water temperature is 12.8-21.0° C, but most activity occurs at or above 15° C. [13]

Mature females may contain 2000-15,000 golden yellow eggs. Males may spawn with several females on a single nest. On average each nest contains about 2,500 eggs, but nests may contain as many as 10,000 eggs. Eggs hatch in about 10 days if water temperatures are in the mid-50's (°F), but can hatch in 2-3 days if temperatures are in the mid-70's (°F). Males guard the nest from the time eggs are laid until fry begin to disperse, a period of up to a month. [14]

Movement: A smallmouth bass movement study in the Yampa River noted that smallmouth bass can move great distances upstream and downstream (Hawkins et al. 2009).

Food: The diet of smallmouth bass changes from small to large food items as the fish grow. Fry feed on microcrustaceans. Juvenile smallmouth bass eat larger insects, crayfish, and fish. Adults primarily feed on fish and crayfish in both lakes and streams. The diet is influenced by abundance and availability of prey. [15]

Risk: In Arizona, smallmouth bass reportedly are responsible for eliminating or reducing some populations of native fishes. [16]

Bass spawning videos:

• Video 1
• Video 2

Large fluctuations in water level can affect reproductive success (Pflieger 1975; Montgomery et al . 1980). Ideal spawning conditions include one or more substantial rises in water level a week or two prior to bass nesting (Pflieger 1975) and relatively stable water levels while nesting is in progress (Watson 1955; Pflieger 1975). Rising water may flush nest areas with cold water, causing nest desertion and halting embryo development (Watt 1959; Montgomery et al. 1980). Falling water levels may drive guarding males off, limit water circulation around eggs, and increase predation, resulting in lower reproductive success (Neves 1975; Montgomery et al. 1980).[17]

Fry seem to be especially vulnerable to flood conditions and fluctuating water levels (Larimore 1975). A rapid drop in water level may trap them in areas where they will become desiccated (Montgomery et al . 1980). A stream rise of only a few inches may displace advanced fry newly risen from the nest (Webster 1954). Most fry remain in shallow water (Doan 1940; Forney 1972), although some may be found at depths of 4.6.-6.1 m (Stone et al. 1954; Forney 1972). Fry 20-25 mm in length cannot maintain themselves in current velocities > 200 mm/sec (Larimore and Duever 1968). An increase in turbulence during flood conditions creates conditions with which smallmouth fry appear unable to cope (Webster 1954). Fry cannot tolerate and are displaced at high turbidities (2,000 JTU) combined with an increase in water velocity, but they will not be displaced at moderate turbidities (250 JTU) (Larimore 1975). Low water temperatures during flood conditions will reduce fry swimming ability (Larimore and Duever 1968).[18]

Lake Powell filled in 1980 after 17 years of constantly rising water levels. The consequences were dramatic. Largemouth bass and crappie had dominated the fishery in the filling reservoir. That changed, as the full reservoir began fluctuating up and down. Annual fluctuating water levels combined with daily wind and wave action uprooted brush in the fluctuation zone. The lake topped out at 3700 feet while lake level changed an average of 20 to 25 feet in any given year. Low water levels in the fall allowed some quick growing annual weeds to survive but perennial plants did not have enough time to become established before spring floods covered the ground once more. The shoreline now consisted of barren rock and sand instead of terrestrial desert vegetation that could be used by young bass and crappie as nursery cover when the flood covered the new brush each spring.

Monitoring the situation would not fix this problem as largemouth bass and crappie populations would crash as a result of inadequate nursery cover. The rapidly expanding striped bass population would incorrectly be blamed for the demise of largemouth bass and crappie. It was time for active management. My research indicated that smallmouth bass would reproduce and perform well in rocky substrate based on the simple principle that smallmouth fry hide in rocks when danger threatens. Largemouth bass and crappie fry must hide in brushy cover to ward off predation. Without brush, largemouth survival was poor but with two thousand miles of rocky shoreline smallmouth bass would thrive. This research was presented to the staff in Salt Lake who approved the concept.

This is where it gets a bit tricky. Our striped bass hatchery in Big Water, Utah was out of production after striped bass natural reproduction was discovered in 1979. The hatchery was not used in 1980 or 1981. But we had a hatchery and the means to grow smallmouth bass. All that was needed was approval from Salt Lake, an adult bass brood source, and the ponds under my direction could turn out smallmouth bass fry in short order. The plans were approved and we went to work. Smallmouth bass were readily available in Flaming Gorge Reservoir. In spring 1982 we grabbed our fishing rods, caught 150 bass, and relocated them from the cold northern Utah border to the Southern Utah tropics.

It takes time to understand all the nuances of raising a new species of fish in a hatchery. The first year is often a learning experience where mistakes are made that lead to correct operations in the coming years. It was no different in 1982. We had not used the hatchery for two years and we had never raised smallmouth bass before, so the learning curve was steep. We made some mistakes but managed to produce about 10,000 bass fry when 10 times that many were expected.

My instructions from Salt Lake were to produce as many bass as possible. The highest priority was to place the first year’s progeny in Starvation Reservoir in Northern Utah. Second priority was to stock another lake in the north and third priority, if we had enough fish, would be Lake Powell.

Obviously with our meager production results there were not enough bass to stock all three lakes. The decision was made to place all the production from 1982 in Starvation Reservoir. The ponds were lowered and the two to three inch fingerlings were harvested over a three day period. It took a long time to harvest fish from our antiquated ponds that did not have catch basins. We had to lower the ponds individually and seine each pond a number of times to collect fish, which were then placed in a small 10x10 ft holding net until all the ponds were drained. On the third day the ponds were all harvested with all smallmouth fry placed in one holding net in one of the ponds. The stocking airplane was scheduled to arrive the next morning.

The plane landed at sunrise and we were dipping small bass out of the large holding net in the pond. Bass fry were dipped from the big net and placed in seven small holding nets. Then the small nets were suspended in a fish tank and rushed a half-mile up the hill to the flat road where the plane had landed. There were just enough fish in each net to fill one of the seven small, wet fish compartments on the plane. These 10,000 fish were extremely crowded in the plane’s holding tank. Time was critical to get them loaded and stocked as soon as possible. The flight took over two hours from Wahweap to Starvation so we worked fast and hard to make it happen. The plane was airborne again in less than an hour.

After the intense and focused effort to load fish in a timely manner it was time to relax and drain the fishpond. As the water went down it became obvious that the pond contained more than a few escapees. I looked at the net and saw that overnight maybe 500 but less than 1000 bass had exited our holding net out of one small hole. The plane was gone and it was not possible to put these fish on that flight. There were not enough fish to justify another expensive cross state flight to Starvation or to another lake in northern Utah. We had a few extra smallmouth bass.

In 1982 there were no cell phones, no Internet, and at the hatchery there was no telephone. We did not even have a hatchery building. We had a hatchery stocking truck and 500 extra fish. Someone needed to make a decision. My first option was to send the fish to Starvation. That goal had been met. The second option was to bring the plane back to put 500 fish in another lake. That was not economically feasible. Option three was to put 500 fish in the stocking truck and drop them off in Lake Powell on the way home. We could meet our third priority and take care of priority two next year when more fish would be raised.

It made sense, and it was economically practical. I stocked smallmouth bass in Lake Powell by driving the hatchery truck down Crosby Canyon road, opening the tube, and letting the fish flow down the pipe. It only took an hour and no extra funds. It was the right thing to do and I didn’t give it an extra thought.

In those days we had much more autonomy. We had to make decisions based on our directions and understanding. We communicated with our superiors by phone and by writing a monthly report, which was sent by US mail. My smallmouth stocking was completed before having access to the phone so I just included that incident in my monthly report and sent the letter. About three days later when the letter arrived in Salt Lake my phone rang shortly after the letter was opened.

The conversation was short. Don Andriano asked, “You did what?” I responded “Yes, I stocked 500 smallmouth bass in Lake Powell as it was the best choice of the options available to me at the time.”

Don asked, “Did you know that we did not have permission to do that?” I responded “No I was never told that, only that Lake Powell was third priority for stocking smallmouth bass this year.”

What was done was final. The conversation was over. I am sure Don had some explaining to do before the Colorado River Fish and Wildlife Council. In retrospect, asking forgiveness may have been a better choice than asking permission, which likely would never have been granted as I later learned with my proposal to introduce rainbow smelt into Lake Powell.

One result of my management action was satisfaction in knowing that smallmouth bass predictably performed magnificently along the rocky shoreline of the lake just as my research indicated they would. Anglers have caught millions of smallmouth bass over the past 30 years.

Another consequence was the threat of smallmouth bass predation on native species in the river running through the Grand Canyon. Fortunately, smallmouth did not populate the river as it was too cold for successful bass reproduction. Bass and native chubs and suckers need water temperatures above 60 degrees to successfully reproduce. The cold tailwater coming out of Lake Powell (46F) does not often provide adequate spawning temperature so bass and native fish do not reproduce in the mainstem Colorado River in most years.

Largemouth and smallmouth bass have been good teammates in Lake Powell. As predicted, largemouth bass numbers crashed shortly after the lake filled. The lake over filled in 1983 covering brush along the shoreline once more. That gave largemouth an extra year and the population boomed until the lake stabilized and gradually declined from 1984 to 1992. When the brush was gone largemouth bass numbers plummeted.

Drought-induced low lake levels in the early 90s allowed more brush to grow on the shoreline. Rising water in 1993 and 1994 covered the new brush and largemouth bass responded immediately as fry produced found nursery cover and survived at a high rate. That population peak lasted from 1994 to 1996 as a generation of bass completed their life cycle. The lake then stabilized again eliminating brush in the fluctuation zone. The next drought occurred at the same time as the Y2K scare that had folks wondering if computers would melt down as the new century was ushered in. From 2000 to 2004 Lake Powell steadily declined to a new low of 3555 MSL. New brush grew along the wet shoreline, as lake bottom became sandy beach once more. When the lake came back up from 2005 to 2008, largemouth bass again peaked in numbers and regained the high population level last seen when the lake filled and overfilled in 1981 to 1984.

I have included this technical discussion and graphics to show that largemouth bass respond to lake level and subsequent brush growth or elimination. Management of largemouth bass in Lake Powell would be possible if lake level could be controlled and regulated to allow new brush growth on demand. If there was a large red button on my desk that allowed me to control lake level, I could produce a consistent trophy-sized supply of largemouth bass and crappie at Lake Powell just by manipulating brush cover.

Some have logically stated that striped bass predation has been responsible for the demise of largemouth in Lake Powell. The data indicates that the recent largemouth population boom from 2007 to 2011 occurred when the striped bass population was also abundantly strong. That seems impossible if striped bass predation was responsible for a decline in largemouth bass abundance. There is not a direct correlation or relationship between striped bass and largemouth populations. They are often separated without much contact by their affinity for different habitat types. Striped bass live in open water and prefer to feed on shad while largemouth live in shallow brushy habitat and feed on shad, sunfish and crayfish.