Difference between revisions of "Smallmouth Bass Page"

PAGE, Ariz. —On July 1, 2022, juvenile smallmouth bass were found in the Colorado River below the Glen Canyon Dam. Smallmouth bass are a predatory non-native fish that have adversely impacted native and federally protected fish in the upper basin of the Colorado River over the last 20 years. The National Park Service (NPS) is working closely with partners and stakeholders to coordinate rapid response actions to address this finding in accordance with previously approved management plans.

Lower lake levels at Lake Powell and rising temperatures in the Colorado River have increased concerns that non-native, warm-water predators, including smallmouth bass, could pass through the dam and begin reproducing. If these warm-water predators establish populations below the dam, it is likely they would negatively impact native fish communities in Glen Canyon National Recreation Area and in Grand Canyon National Park.

The NPS encourages anglers to report any observations or catches of smallmouth bass below the dam. If caught, please remove the fish, put on ice and contact staff at Glen Canyon National Recreation Area or Grand Canyon National Park with a description of the location where found. Report findings to: Jeff Arnold, Glen Canyon National Recreation Area, e-mail us or Brian Healy, Grand Canyon National Park, e-mail us. [2]

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. [3]

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. [4]

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. [5]

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. [6]

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.[7]

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).[8]

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). [9]

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. [10]

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. [11]

Risk: In Arizona, smallmouth bass reportedly are responsible for eliminating or reducing some populations of native fishes. [12]

• Nonnative Invasive Aquatic Species
• Habitat suitability information: Smallmouth bass
• 2019 National Park Service Expanded Non-native Aquatic Species Management Plan
• 2013 National Park Service Comprehensive Fisheries Management Plan

• Evaluate effects of flow spikes to disrupt reproduction of smallmouth bass in the Green River downstream of Flaming Gorge Dam

2022

• 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

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).[13]

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 dessicated (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).[14]