Paula Marcinek and Peter Dimmick scan robust redhorse for PIT tag. Photo credit: Alan Cressler.

Studying fish and other aquatic species is often difficult, especially if the species you’re searching for is rare and access to habitat is limited. Sampling in aquatic environments also almost always requires specialized equipment, and even more so when the target lives in waters that require using a boat.

The threatened robust redhorse sucker is a prime example. In 1991, while conducting standard game-fish surveys with a modified boat in the Oconee River downstream of Sinclair Dam, DNR fisheries biologists collected an unusual sucker. It turned out to be a robust redhorse, a species that had been considered extinct. This rediscovery sparked interagency efforts to conserve the fish, efforts that continue today.

Interestingly, the small electrofishing boat used to collect that first robust redhorse nearly 30 years ago is still in use. More on that in a minute. At least as interesting, though, is how electrofishing works.


Catching with Current

BoatElectrofishingIllustration_See Citation

Boat Electrofishing Illustration. See citation at end of article.

Electrofishing creates an electric current in the water which draws fish toward it in a process called galvanotaxis – the movement of an organism or its parts in a specific direction in response to an electric current. That current momentarily stuns and immobilizes the fish, making them easier to net.

When electrofishing is done properly, fish revive quickly — sometimes too quickly for capture! – suffer no harmful effects and are released soon after processing. The processing varies depending on the project. It usually includes identification and counts. Catches of special interest, such as a robust redhorse, are tagged with a unique identifier and their sex, gravidity, weight and length are recorded. If a tagged fish is recaptured, the data can be used to estimate growth and movement.

Electrofishing the Oconee River.

Electrofishing the Oconee River.

The water’s conductivity is an important factor in electrofishing. Conductivity is related to the concentrations and types of dissolved ions in the water such as salts and inorganic materials (for example, chlorides, sulfides and carbonate compounds). Ions in waterways come from natural sources – including eroding rocks and groundwater – as well as from human sources, such as wastewater discharges and agricultural runoff. Water running off insoluble bedrock will have low conductivity, whereas runoff from highly fertilized farmlands will have high conductivity.

The specialized system of a shock-boat begins with a gas-powered generator that powers a control box. The control box maximizes the system’s efficiency across the varying conductivities of each site. It converts the energy into a form that can be adjusted for conditions and targeted species. Current is channeled through wiring to a pair of booms off the front of the boat. The booms are outfitted with short, metal cables dangled under the water. The current travels from the cables through the water to the bow of the boat, creating an electric field that extends several feet out from and under these “probes.”


Overhauled for Science

The sampling team consists of a person who operates the boat and control box, plus one or two people with nets in the bow. The operator moves the boat slowly through a survey area, directed by the netters who are eyeing the water. All team members wear polarized glasses to reduce surface glare and allow them to better spot fish and navigational hazards such as stumps.

When stunned fish float to the surface, the netters lean over an insulated railing and safely scoop them up. Fish are processed immediately or held in an aerated live-well for a short time.

Original jon boat used to during discovery of robust redhorse in the Oconee River.

Updated version of the jon boat used to during discovery of robust redhorse in the Oconee River.

Now about that jon boat used to rediscover robust redhorse. To continue efficiently sampling for these fish, the aquatics team of DNR’s Wildlife Conservation Section has upgraded the old but tough boat. The changes include a new generator, boat motor and repairs to the electrical system and booms. Because much of the habitat where this boat is used is inaccessible by existing ramps, staff also modified the trailer so the boat can be launched off river banks.

These critical improvements were funded through a federal Competitive State Wildlife Grant that supports monitoring, assessment and restoration efforts for the robust redhorse.

The upgrades also mean more time on the water – and more robust redhorse monitored – for this little electrofishing boat that could.

Paula Marcinek, Deb Weiler and Peter Dimmick are part of the Wildlife Conservation Section’s aquatics team.

Citation for electrofishing illustration: Imbriw (2013). Inland Waters Fish Monitoring Operations Manual: Electrofishing Health and Safety/HCMR Rapid Fish Sampling Protocol. Hellenic Centre for Marine Research – HCMR Special Publication, draft version 1. 79 pp.