Why Electrofishing Can Miss Big Largemouth Bass
Fisheries biologists use boat electrofishing to estimate how many bass live in a lake and how large they are. But a new study suggests the largest fish are easier to miss than many anglers might assume. In two Oklahoma hatchery reservoirs, bass in the 510–595 mm (about 20–23.4 inch) length class had markedly lower electrofishing catchability than mid-sized bass. The researchers also observed fish reacting to an electric field before the boat could immobilize them.
Research snapshot: Hull and colleagues, North American Journal of Fisheries Management, published October 23, 2026. Read the peer-reviewed study (DOI). This article summarizes that research, not a universal rule for every bass fishery.
What did the researchers find?
The team worked with marked black bass in two hatchery water-storage reservoirs at the Durant State Fish Hatchery in Oklahoma. They used spring and fall electrofishing trials, tagged fish, radio telemetry, live-imaging sonar and comparisons with angler catches to test whether electrofishing samples different bass sizes equally well.
The clearest pattern was lower catchability for the largest length class. Mean catchability, expressed as recaptures per marked fish per hour of sampling effort (q), varied by fish size:
| Length class | Approximate length | Mean catchability (q) |
|---|---|---|
| Stock–quality | 200–299 mm (7.9–11.8 in) | 0.18 |
| Quality–preferred | 300–379 mm (11.8–14.9 in) | 0.28 |
| Preferred–memorable | 380–509 mm (15.0–20.0 in) | 0.24 |
| Memorable–trophy | 510–595 mm (20.1–23.4 in) | 0.11 |
Source: Hull et al. (2026), study abstract and results. Length conversions are rounded. The values above are study-specific catchability coefficients, not the probability that an individual fish will be caught by one pass of an electrofishing boat.
In practical terms, the 20-inch-and-larger class had less than half the measured catchability of the approximately 12–15-inch class (0.11 versus 0.28). That difference can distort the apparent size structure of a population when managers rely on uncorrected catch-rate data.
Mean catchability (q) by fish length in four size classes. Bars are scaled to 0.28, the largest estimate.
| Bass length | Relative bar | q |
|---|---|---|
| 200–299 mm | 0.18 | |
| 300–379 mm | 0.28 | |
| 380–509 mm | 0.24 | |
| 510–595 mm | 0.11 |
Visualized by Bass Facts from Hull et al. (2026). The values are rate-based catchability estimates, not percentages. They come from two hatchery reservoirs, not a universal lakewide survey. Study source.
How might big bass escape electrofishing?
Live-imaging sonar provided an important clue. Researchers observed some bass changing direction or escaping near the outer edge of the electrical field, before they entered the more intense portion where immobilization becomes possible. Fish avoided the approaching boat at greater distances when the electrofishing system was powered on than when it was off. Those observations are consistent with detection and avoidance of the field.
Habitat may also matter. Larger fish in this study tended to occupy deeper or more offshore areas. Those fish may be less accessible to a sampling approach that is often most effective near shore. The study supports multiple contributing explanations; it does not establish that every large bass detects and escapes an electrofishing boat.
Does this mean lakes have more trophy bass than surveys show?
Potentially—but not necessarily. The results demonstrate a source of sampling bias: a survey can underrepresent large fish even when those fish are present. The authors showed that applying catchability corrections could substantially change estimated size-distribution metrics in their study reservoirs.
It would be a mistake, however, to apply one universal multiplier to every lake. The field work involved two hatchery reservoirs, and relatively few of the marked bass were in the largest sizes. Electrofishing settings, habitat, season, water clarity, water conductivity, species composition and fish behavior can differ elsewhere. Reliable management still requires local survey methods and professional interpretation.
The researchers compared electrofishing catches with angler catches, but angler-caught fish are not an unbiased census either. Anglers choose places, presentations and fish sizes differently from fisheries crews.
Why it matters for bass anglers
When a wildlife agency reports a low number of 20-inch bass from an electrofishing survey, that figure describes what the survey caught, not automatically every large bass that lives in the reservoir. Survey trends can still be useful, especially when methods are consistent, but catchability is part of the interpretation.
The finding also connects to a broader theme in bass science: the fish we can observe and catch are not necessarily a representative sample of all the fish present. Habitat and animal behavior influence the result.
What is established, and what remains uncertain?
- Supported in this study: larger bass had lower electrofishing catchability than mid-sized bass; sonar observations indicated avoidance near the energized field; depth and offshore position may add to the bias.
- Not established: a nationwide estimate of hidden trophy bass, a correction factor valid for every reservoir, or proof that every individual bass responds the same way.
- Practical implication: fishery assessments should consider size-dependent capture efficiency before interpreting length distributions or extrapolating mortality estimates.
Primary source and further reading
Hull, Nathanael J., and coauthors (2026). “Understanding size bias in electrofishing samples of black bass.” North American Journal of Fisheries Management. Published October 23, 2026. https://doi.org/10.1093/najfmt/vqag095.
Explore related Bass Facts resources: How fast largemouth bass grow, how reservoir conditions shape bass habitat, and the Texas Giant Bass Data Hub.
Research reviewed November 4, 2026. The article distinguishes reported results from broader inference and will be revised as additional field studies become available.