Electrofishing teams must account for overlapping electrical fields

Electrofishing is indispensable to many river surveys because it allows fish to be captured, measured and released where conventional nets may be inefficient or more harmful. “Non-lethal”, however, is not the same as harmless. A field study from Idaho’s Big Wood River found spinal abnormalities that would not necessarily be visible during routine handling. It also documented a system-level hazard: three backpack electrofishers, each programmed at 60 Hz, produced effective pulse frequencies of 120 or 180 Hz when their electrical fields overlapped.

The study does not support abandoning electrofishing, nor does it establish a universal safe setting. Its practical message is that risk belongs to the complete field configuration—equipment count, synchronisation, spacing, water conductivity and crew movements—not simply to the number displayed on each control box.

Testing two realistic survey configurations

Kevin Meyer and colleagues worked in September 2024 along a 12-km reach of the Big Wood River in central Idaho. The site was 1,800–1,900 m above sea level, approximately 15 m wide at base flow and had a gradient close to 1%. Conductivity was about 120 µS/cm and water temperature ranged from 12 to 15°C. Roughly 6 km were electrofished, with each section sampled only once to reduce the likelihood of fish being exposed to both equipment types.

Before electrical sampling began, the team invested nearly 100 angling hours in obtaining reference fish. Thirty-one Rainbow Trout (Oncorhynchus mykiss) and five Mountain Whitefish (Prosopium williamsoni) were radiographed. None showed an acquired spinal injury, although one trout had two fused vertebrae classified as a congenital defect.

The first electrofishing configuration involved three backpack operators moving upstream in tandem. Each unit was set to 60 Hz, a 24% duty cycle and 300 V, producing approximately 100 W of average power and 0.25–0.4 A. The second used a 3.7-m barge with two anodes connected to the same power source. It operated at 60 Hz, 24% duty cycle and 400 V, generating 300–800 W and 0.8–2.0 A depending on water depth.

Although the barge delivered substantially more power, its two anodes were synchronised. Oscilloscope measurements showed no pulse multiplication when their fields met. The independently operating backpack units behaved differently: overlap between two or three fields generated measured frequencies of 120 or 180 Hz. Operators sometimes moved close together or encircled productive habitat, as they would during an ordinary survey.

Radiography exposed injuries that handling could miss

The electrofishing sample comprised 395 Rainbow Trout measuring 180–504 mm and 128 Mountain Whitefish measuring 205–483 mm. Each animal was measured and imaged in dorsal and lateral views using portable digital radiography. Two readers, blinded to fish length and capture configuration, classified images as normal or showing vertebral compression, misalignment or fracture.

Mean spinal injury prevalence was 44% in Rainbow Trout and 13% in Mountain Whitefish. Across both species, prevalence was similar between barge and backpack capture—34% and 39%, respectively. Equipment category was therefore not the strongest statistical predictor; species and body length mattered more. In trout, the highest prevalence occurred among fish 250–350 mm long rather than increasing continuously with size.

Injured trout most often showed misalignment (62%), followed by compression alone (32%) and fracture (5%). The pattern differed in Mountain Whitefish: 54% compression alone, 38% fracture and 8% misalignment. Some changes were subtle. Twenty-one per cent of recorded misalignments were slight spinal curves without an obviously displaced vertebra, while the authors acknowledge that hairline fractures may not have been visible on the radiographs.

These percentages are not mortality estimates. The study examined skeletal condition soon after capture; it did not measure pain, swimming performance, healing or long-term survival. Minor radiographic changes may resolve, whereas more serious damage may impair movement without an immediate external sign. Mountain Whitefish had fewer skeletal injuries but appeared more vulnerable to handling, with gill bleeding observed in the field. Welfare cannot therefore be represented by one endpoint alone.

A persuasive mechanism, but not individual proof

The similar injury prevalence associated with a high-powered barge and lower-powered backpacks was unexpected. The authors’ leading explanation is that some fish in the backpack operation encountered the measured 120- or 180-Hz conditions when operators converged. More frequent electrical stimulation could produce muscular contractions capable of compressing or displacing vertebrae.

That interpretation remains an inference. The team did not record each fish’s distance from an anode, orientation, exposure duration or actual pulse count. Nor did it determine the separation needed to prevent overlap. An injured fish therefore cannot be linked retrospectively to a specific 120- or 180-Hz event. Netting speed, proximity to the electrode, water depth and orientation in the field are plausible additional influences. Indeed, the best statistical models explained only 16–17% of injury variation.

An earlier experiment in small streams provides essential context. With one backpack electrofisher, constant average power and a fixed duty cycle, 30 and 60 Hz each produced a 4% spinal injury prevalence among samples of 230 and 222 trout. The 60-Hz setting improved cumulative capture efficiency from 0.84 to 0.94. This is not necessarily a contradiction: those streams were only 1–6 m wide, generally held smaller trout and did not require three independent electrical fields.

The combined evidence warns against treating frequency in isolation. A setting that performs acceptably with one operator in a narrow stream may create a different exposure when several units work side by side in a wider channel.

Managing electrofishing as a team procedure

Before a multi-unit survey, teams should verify the signal produced in the water under realistic working positions. An oscilloscope or equivalent technical assessment can reveal pulse multiplication. The written procedure should address operator spacing, possible synchronisation, movements around high-value habitat and what to do when anodes converge.

Lower frequency may reduce injury, but the 2025 study did not test the proposed 20–30-Hz range. Reducing frequency can also lower capture efficiency and bias abundance or occupancy estimates. A pilot should therefore assess both animal outcomes and sampling performance under the conductivity, depth, species composition and equipment configuration expected at the site.

Exposure duration matters as well. Fish should be netted and removed from the field promptly, while holding containers require appropriate water exchange or oxygenation. Field records should include temperature, conductivity, voltage, frequency, duty cycle, power, number of active anodes and any overlap or equipment anomaly. Stop criteria should cover mortality, gill bleeding, abnormal swimming and prolonged recovery. Where surveys are repeated, radiographic or other targeted welfare audits on a justified subsample may uncover damage that external examination misses.

The evidence should not be overgeneralised. This was one river, two salmonid species and a single set of electrical configurations. Fish were also exposed to relaxation, measurement and radiography after capture, and long-term consequences were not evaluated. Further work is needed to test lower frequencies with multiple backpacks while preserving reliable population estimates.

Vetofish can support environmental monitoring teams with risk assessment, procedure design, operator training and welfare indicators tailored to the species and site. The central lesson is operational: safe electrofishing is not merely a machine setting. It is the outcome of how every unit, operator and fish interacts within the same electrical field.

References

• Meyer K. A., Reynolds J. B., Frawley S. E. et al. (2025). “Spinal injuries in Rainbow Trout and Mountain Whitefish captured in a wadeable river using backpack- and barge-mounted electrofishers.” North American Journal of Fisheries Management, 45(6), 1108–1115. https://doi.org/10.1093/najfmt/vqaf087
• Idaho Department of Fish and Game (2025). Wild Trout Evaluations — Annual Progress Report, Subproject 4, pp. 58–76. Full primary report
• Chiaramonte L. V., Meyer K. A., Branigan P. R., Reynolds J. B. (2020). “Effect of Pulsed DC Frequency on Capture Efficiency and Spinal Injury of Trout in Small Streams.” North American Journal of Fisheries Management, 40, 691–699. https://doi.org/10.1002/nafm.10440