MDNR Report

Electrofishing a Valuable Tool to Fish Managers

On a rainy August morning, three men dressed in dark-shaded green chest waders and rain jackets slowly make their way up the chilly waters of the Rock River in Alger County.

Two of the men carry long white poles with boxed ends in one rubber-gloved hand and a fishing net in the other.

From each of the two men, a yellow electrical cord runs downstream to an equipment box in a small aluminum boat, which is being pulled up the river by the third man.

As the men wave the white poles under the stream banks and overhanging trees, large and small brook trout emerge, floating sideways or upside down in the creek.

Quickly, the fish are netted and moved to a plastic bin filled with water in the bottom of the boat. The men pull the boat to the shore and begin measuring fish and collecting information on each of them.

They work quickly to release the fish safely back into the stream.

This process is called electrofishing, performed by state fisheries biologists and technicians.

The Fisheries Division is responsible for managing fish populations within the states streams, protecting and preserving these valuable resources for the public and posterity.

Because every stream is unique, each one requires attention to factors that may, or may not, have an impact on the fish community.

In every case, fisheries managers are trying to gather more data on fish populations to help guide management recommendations.

How it works

Electrofishing in Michigan got its start at the Hunt Creek Research Station in Montmorency County during the summer of 1942.

There, the electrofishing unit first used in Michigan was a 1-horsepower gasoline motor that powered a 500-watt generator. The electrical current was conducted to the water through a rubber-covered, two-wire cable using a pair of electrodes that were attached to wooden handles. The two electrodes used the water to complete the electrical circuit to the generator.

Today the concept is still the same, although the equipment has come a long way since 1942. There are three main electrofishing tools used by the DNR in Michigan.

The first is called a backpack shocker. It is designed for small streams and is very portable. It runs off a 12-volt battery or sometimes a small generator.

The second type is called a stream shocker. These units are designed for larger streams that a backpack shocker cannot cover adequately, and they are what the fisheries team was using on the Rock River that rainy day in August.

A stream shocker is made up of a small generator and a control box that can alter the amount of current being produced by the generator. The electric current flows from a positive to negative charged direction.

Electrofishing a Valuable Tool to Fish ManagersThe final, and largest, electrofishing tool is the boom shocker. These tools are used on large rivers and lakes. They usually consist of a 16- to 20-foot, flat-bottomed boat equipped with large booms, a generator and a control box. The booms act as the anode (positive) electrode, and the boat serves as the cathode (negative) electrode.

All three types of electrofishing gear options serve the same purpose. They produce an electric current in the water which temporarily stuns the fish, firing their muscles involuntarily, allowing technicians to collect the fish and collect biological samples.

The fish are netted and placed in temporary holding tanks to be sampled and then released back into the water of the stream or lake being surveyed.

For decades, fisheries managers have used electrofishing gear to grow their knowledge about the fish community that lives in each stream or lake.

The many uses for electrofishing gear include:

• Aiding in estimating the number and type of species living within fish communities.
• Collecting wild fish for egg gathering. The eggs are taken to fish hatcheries and hatched. The fish produced from these eggs are used for stocking streams and lakes.
• Providing data to help judge the effectiveness of fisheries management actions.
• Monitoring important fish species or non-native, invasive species that can harm fish populations, water quality, recreation or economic concerns.

In the early 2000s, the DNRs Fisheries Division developed a standard process for stream sampling to compare fish populations between different streams with similar habitat types. Electrofishing gear is used to collect information from the populations in these streams.

Biologists are gathering information on length, fish species type and age. They often will take scale or spine samples to help determine age.

The Rock River is sampled every three years and contains a wild population of brook trout.

By sampling within a 1,000-foot length of stream, a population estimate for brook trout is calculated from each sampling effort. From the information gathered, trends in brook trout abundance, mortality and growth can be identified, which are key components to fisheries management.

Each spring the DNR electrofishing boats head to the Muskegon River, below Croton Dam, in search of walleye.

The Muskegon River serves as one of two locations in Michigan where wild walleye brood stock (eggs used to grow fish populations) is collected. The second is Little Bay de Noc in Delta County.

Because of the high and fast water conditions of the Muskegon River during the spring spawning run, electrofishing boats are very effective at successfully capturing spawning walleye. This provides the number of eggs needed to meet targets for production at the Wolf Lake State Fish Hatchery in Van Buren County.

Additionally, about one month later, on the east side of Michigan, electrofishing boats search the Detroit River for muskellunge during their spawning period. Great Lakes, or spotted, muskellunge have been the primary focus for DNR Fisheries Division production since 2010.

Until inland brood-stock lakes become well-established with an acceptable population level of Great Lakes muskellunge, the DNR will rely entirely on the Detroit River to collect eggs for its statewide muskellunge stocking program.

In the early stage of this muskellunge program, efforts with other gear types were unsuccessful on this massive waterbody. Today the sole method for collection is with electrofishing boats.

When someone reports seeing an invasive species, field staffers often conduct electrofishing surveys to determine the presence or absence, and location, of the invasive species.

With the development, refinement and continued use of this technology, questions once thought to be impossible to answer are now able to be answered efficiently and confidently.

Electrofishing is a tool likely to remain important in helping to protect and preserve Michiganís thriving stream and lake fish communities well into the future.