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Black Bass · Centrarchidae

Smallmouth Bass

Micropterus dolomieu

A crayfish-eating river predator with hard site fidelity, a contested spawning-season biology, and a taxonomy that was rewritten in 2022.

Smallmouth Bass (Micropterus dolomieu)
Smallmouth Bass. Raver, Duane, Public domain. Source.

Name and taxonomy

The currently accepted scientific name is Micropterus dolomieu. The older spelling dolomieui, which appears in a great deal of twentieth-century literature and on a great many signs, is not the accepted form. Both honor Déodat Gratet de Dolomieu, the French geologist the species was named for in 1802.

The genus was substantially rewritten in 2022. Kim, Taylor and Near’s “Phylogenomics and species delimitation of the economically important Black Basses (Micropterus)”, published in Scientific Reports (volume 12, article 9113), used genome-scale ddRAD data to delimit nineteen species of Micropterus. Two consequences matter directly for rivers.

First, the Neosho smallmouth bass of the Arkansas River drainage in Missouri, Arkansas, Oklahoma and Kansas was elevated from subspecies to full species status as Micropterus velox, the Neosho Bass. Second, the study identified two further lineages within what had been called smallmouth bass — the Ouachita Bass of the upper Ouachita River system and the Little River Bass — as distinct species that remain formally undescribed, carried in the literature as M. cf. dolomieu. In practical terms, a “smallmouth bass” caught in parts of the Ozarks and Ouachitas may not be M. dolomieu at all, and the boundaries are still being worked out.

Recognizing the fish

Smallmouth are elongated, laterally compressed bass with a bronze to olive-brown back, a pale belly, and a pattern of broken vertical bars along the flank rather than the horizontal stripe of a largemouth. The eye is typically red or amber. The Missouri Department of Conservation gives the two structural characters that settle any ambiguity: very small cheek scales, and connected dorsal fins — the spiny and soft portions joined rather than nearly separated. The upper jaw “extends to about the rear margin of the eye” but no further, whereas in largemouth it extends past the eye. The Pennsylvania Fish and Boat Commission describes the same characters in shorthand: brown or bronze back, pale yellow belly, “vertical, broken bars along each side of the body,” and the upper jaw stopping at the back edge of the eye.

River fish are typically darker and more heavily barred than lake fish, and fish over hard, clean rock run brighter bronze than fish over silt.

Rivers, current and structure

Smallmouth are a current-oriented species. In Ozark streams they are “the dominant species of large, predatory fish in most streams,” occupying habitats with “silt-free rock or gravel bottoms, near riffles but not in the main current.” That last phrase is the whole pattern in miniature: the fish sit in the seam beside fast water, not in it, using the current to deliver food while paying as little swimming cost as possible. Pennsylvania’s description matches — “warm streams and rivers with gravel bottoms” and “rocky areas along weed beds.”

Silt is the limiting factor across most of their range. Smallmouth need interstitial space in coarse substrate for crayfish, for hellgrammites, and for spawning gravel. Watersheds that deliver heavy fine sediment lose smallmouth even where temperature and flow remain suitable.

Water temperature

Water temperature governs the smallmouth year more tightly than any other variable. Feeding activity climbs steeply through the sixties, peaks somewhere in the seventies, and falls off below about 50 °F, when fish move to deep wintering pools and slow markedly. Published optimum ranges differ between agencies and study systems; the figure that is consistently documented is the spawning threshold, which Missouri places at water temperatures exceeding 60 °F.

Two thermal stories are worth knowing. The first is refuge. In the Missouri Ozarks, enormous karst springs buffer river temperature year-round. Ozark National Scenic Riverways describes springs that “deliver billions of gallons of clear, cold water to the Current and Jacks Fork River every week,” with Big Spring alone discharging roughly 289 million gallons a day at stable year-round temperature after a subsurface journey of up to 45 miles. Below such an inflow the river runs cold enough to concentrate fish in summer and to shift the whole thermal profile of the reach downstream.

The second is expansion. Warming water opens new river to smallmouth, sometimes with severe consequences. The US Geological Survey’s Grand Canyon Monitoring and Research Center records that in 2022, low reservoir levels meant water from Lake Powell’s warm upper layer was released downstream, “dramatically warming river temperatures and carrying smallmouth bass through the dam into the Colorado River,” producing the first records of smallmouth reproduction below Glen Canyon Dam. USGS calls invasive smallmouth “the greatest threat to native fishes” in that system, noting they have been linked to humpback chub declines including one population that was rapidly extirpated.

Diet: crayfish first

Crayfish dominate riverine smallmouth diets by volume wherever both occur. Missouri lists the diet as “fish, crayfish, and large aquatic insects,” and describes a distinctive foraging habit — following turtles or suckers to capture prey those animals dislodge from the bottom. Pennsylvania’s summary of what smallmouth eat in rivers is similarly crayfish-first, followed by minnows and hellgrammites.

The diet changes with size and season. Dauwalter and Fisher’s study of ontogenetic and seasonal diet shifts in an Ozark stream documents the transition from invertebrate feeding in small fish to crayfish and fish in larger ones. The USGS notes that introduced smallmouth “experience a niche shift, switching food preference from insects and zooplankton to crayfish and fish, that causes competitive interactions with many other fish species” — the same shift that makes them a formidable native predator makes them a destructive introduced one.

Invasiveness and salmon

The US Fish and Wildlife Service Ecological Risk Screening Summary scores smallmouth bass History of Invasiveness: High, Climate Match: High across virtually all of the contiguous United States, and Overall Risk: High, with high certainty of assessment given the volume of peer-reviewed literature. The listed impacts include “hybridization with native Micropterus sp., changes in invertebrate communities, and native fish species population reductions and extirpations,” plus the potential to carry and spread viral hemorrhagic septicemia virus.

The clearest documented conflict is with Pacific salmon. The USGS Nonindigenous Aquatic Species factsheet states that smallmouth “have been shown to eat smolts of Pacific salmonids, therefore posing a threat to these already declining species in the Columbia River,” and records smallmouth “consuming up to 35 percent of outmigrating wild salmon.” The magnitude has been quantified repeatedly, including Tabor and colleagues’ work on bass predation on juvenile Chinook salmon and Tiffan and colleagues’ assessment across a broad river continuum. A contributing factor frequently raised in this literature is that juvenile salmon evolved with a different set of predators, and their innate anti-predator responses to an introduced predator are weaker than to native ones — a plausible and much-discussed mechanism, though the strength of the evidence for it varies by study system.

Spawning and the nest-guarding male

Smallmouth spawn in spring. Missouri records spawning “from early or mid-April” once water passes 60 °F, continuing into June or July with a peak in May. The male excavates “a saucer-shaped depression” on gravel or rock, usually in a protected pocket out of the main current, and after spawning he guards the eggs and then the fry, fanning silt off the nest and driving off predators, until the fry disperse roughly two weeks after hatching. William Pflieger’s classic account of smallmouth reproduction in a small Ozark stream remains one of the best descriptions of the sequence.

That guarding period is the species’ structural vulnerability. A guarding male is tied to a fixed point, is aggressive toward anything that approaches, and cannot be replaced if he leaves. Remove him for even a short time and nest predators — sunfish, crayfish, other bass — can strip the brood. Steinhart and colleagues documented the interaction between storms, angling and nest predation during angling in Canadian Journal of Fisheries and Aquatic Sciences.

What the closures showed, and what remains contested

The COVID-19 pandemic produced an unplanned natural experiment. Philipp, Zolderdo, Lawrence, Claussen, Nowell, Holder and Cooke reported it in “COVID-19 reduced recreational fishing effort during the black bass spawning season, resulting in increases in black bass reproductive success and annual recruitment” (Fisheries Research, volume 259, article 106580, 2023). On Lake Opinicon in Ontario, where nesting bass had been monitored for decades, the sharp drop in spawning-season angling effort coincided with nest success rising roughly threefold — from under a quarter of nests to around two-thirds — and with two to three times as many independent fry produced.

Whether that translates into more adult fish is where the field genuinely splits. The counter-position, held by a number of state agencies and researchers, is that population-level effects of spawning-season angling have not been demonstrated in healthy populations: recruitment in bass is typically limited later in the first year by density-dependent processes, so a larger fry cohort need not produce a larger year class. That disagreement has been argued directly in the primary literature, including a published comment by Parkos, Wahl and Philipp in Transactions of the American Fisheries Society responding to work that found limited population consequences. Both positions rest on real data; they differ about which stage of the life cycle sets the ceiling. This article does not attempt to resolve it, and this site does not restate any jurisdiction’s rules — those live with the agency.

Site fidelity and movement

Smallmouth show strong attachment to particular places. Wiegmann, Weinersmith, Baylis, Newman and Angeloni’s “Null models confirm nest site fidelity by male smallmouth bass” (BMC Zoology, 2024) used a decade of nesting data and compared observed inter-nest distances against simulated random nest placement. Repeat-breeding males nested a median of roughly 46 to 120 metres from their previous year’s nest across eight consecutive year pairs, and between 6 and 18 percent of males used the identical nest in consecutive years — distances “in all cases much closer than those simulated.” That study was conducted on a Wisconsin lake, but the pattern it confirms is the same one river biologists describe: seasonal movements to and from wintering pools and spawning areas, followed by return to a familiar reach.

Riverine movement has been mapped in detail for the Ozark forms. Brewer and Miller’s assessment of spawning movement and habitat needs of riverine Neosho Smallmouth Bass and Miller, Mollenhauer and Brewer’s work on movement and diel habitat use of juvenile Neosho Smallmouth Bass in an Ozark stream both describe fish operating within bounded home reaches rather than ranging freely. For a river angler, the practical consequence is that a good pool tends to be good repeatedly, and that a fish removed from a productive lie leaves a vacancy another fish will fill.

Angling as natural history

Almost everything characteristic of smallmouth angling follows from the biology above. The fish holds in current seams beside fast water, so that is where they are caught. Crayfish dominate the diet, so crayfish-shaped and crayfish-colored presentations dominate the tackle. Fish are territorial and site-faithful, so productive water repeats across seasons and years. Feeding responds steeply to temperature, so a live water temperature reading predicts activity better than a calendar date. And because a guarding male is anchored to a visible nest and will strike almost anything that comes near it, the spring period is the one where angler behavior and fish biology intersect most directly — which is precisely why the literature on it is contested rather than settled.

Where to catch it

537 waterways on GuidedFloats list smallmouth bass, across 44 states. Each page carries live gauge readings, access notes, and the guides who fish it.

Alabama 2

Arizona 13

Arkansas 15

California 14

Colorado 6

Connecticut 12

Delaware 3

Georgia 2

Hawaii 7

Idaho 9

Illinois 16

Indiana 16

Iowa 15

Kansas 8

Kentucky 16

Maine 16

Maryland 14

Massachusetts 9

Michigan 13

Minnesota 14

Missouri 25

Nebraska 14

Nevada 5

New Hampshire 16

New Jersey 6

New Mexico 7

New York 20

North Carolina 17

North Dakota 11

Ohio 20

Oklahoma 13

Oregon 10

Pennsylvania 25

Rhode Island 6

South Carolina 4

South Dakota 8

Tennessee 17

Texas 5

Utah 2

Vermont 17

Virginia 24

Washington 5

West Virginia 20

Wisconsin 20

References

  1. Smallmouth Bass (Micropterus dolomieu) — Nonindigenous Aquatic Species factsheet — U.S. Geological Survey
  2. Phylogenomics and species delimitation of the economically important Black Basses (Micropterus) (Kim, Taylor & Near, Scientific Reports 12:9113, 2022) — Scientific Reports
  3. Ecological Risk Screening Summary: Smallmouth Bass (Micropterus dolomieu) — U.S. Fish and Wildlife Service
  4. COVID-19 reduced recreational fishing effort during the black bass spawning season, resulting in increases in black bass reproductive success and annual recruitment (Philipp et al., Fisheries Research 259:106580, 2023) — Fisheries Research
  5. Smallmouth Bass — Field Guide — Missouri Department of Conservation
  6. Bass — Catch PA Fish — Pennsylvania Fish and Boat Commission
  7. Smallmouth Bass Expansion Downstream of Glen Canyon Dam — U.S. Geological Survey, Southwest Biological Science Center
  8. Grand Canyon Monitoring and Research Center — U.S. Geological Survey, Southwest Biological Science Center
  9. Null models confirm nest site fidelity by male smallmouth bass, Micropterus dolomieu (Wiegmann et al., BMC Zoology, 2024) — BMC Zoology
  10. Springs — Ozark National Scenic Riverways — National Park Service
  11. Ontogenetic and Seasonal Diet Shifts of Smallmouth Bass in an Ozark Stream (Dauwalter & Fisher, Journal of Freshwater Ecology, 2008) — Journal of Freshwater Ecology
  12. Effects of storms, angling, and nest predation during angling on smallmouth bass reproductive success (Steinhart et al., CJFAS, 2005) — Canadian Journal of Fisheries and Aquatic Sciences
  13. Comment: Assessing the Potential for the Angling of Nesting Males to Influence Black Bass Recruitment (Parkos, Wahl & Philipp, TAFS, 2013) — Transactions of the American Fisheries Society
  14. Assessing the spawning movement and habitat needs of riverine Neosho Smallmouth Bass (Brewer & Miller, Cooperator Science Series, 2022) — U.S. Fish and Wildlife Service Cooperator Science Series
  15. Movement and Diel Habitat Use of Juvenile Neosho Smallmouth Bass in an Ozark Stream (Miller, Mollenhauer & Brewer, NAJFM, 2018) — North American Journal of Fisheries Management
  16. Smallmouth Bass and Largemouth Bass Predation on Juvenile Chinook Salmon and Other Salmonids (Tabor et al., NAJFM, 2007) — North American Journal of Fisheries Management
  17. Impact of smallmouth bass predation on subyearling fall Chinook salmon over a broad river continuum (Tiffan et al., Environmental Biology of Fishes, 2020) — Environmental Biology of Fishes
  18. Reproduction of the Smallmouth Bass in a Small Ozark Stream (Pflieger, American Midland Naturalist, 1966) — The American Midland Naturalist
  19. IGFA World Records database — International Game Fish Association