Electrofishing helps measure water quality
MANKATO — Between buffer strips and invasive species, water quality in Minnesota is an issue that affects virtually every resident.
But how does the state measure water quality? A shockingly simple answer is: electrofishing.
“Electrofishing is a common method that we fisheries biologists use to sample fish populations,” Department of Natural Resources (DNR) Minnesota River specialist Anthony Sindt said.
By counting fish, the DNR can get estimates of how many fish and what species reside in a body of water.
“In a general sense, the more diverse, the more species there are, the healthier the system,” Sindt said.
Electrofishing is a popular method. Sindt recently went surveying on the Minnesota, at the spot where the Blue Earth River flows into it.
He dropped a silver boat that looked similar to a flat-bottomed fishing boat, with a guard rail on the bow and two strange contraptions.
The two long poles that swing out in front of the boat, dangling what looks like a metal asterisk, are called “droppers.” Six rods jut from a short metal cylinder that hangs from a chain. A metal cable runs from the end of the pole, through the cylinder, along and through the end of each rod, dangling into the water.
This works as the anode for the circuit, Sindt said. Electrofishing works by running a high-voltage current through the water from the anode to the cathode. In this case the boat hull served as the cathode.
The electrical field generated attracts the fish, Windom area supervisor Ryan Doorenbos said.
When the fish get too close, they get stunned. Once a fish a stunned, it tends to turn on its side and floats.
This method takes at least two people. Sindt piloted the boat, while Doorenbos and Windom fisheries technician Luke Rossow used dip nets to nab fish.
Launching from the landing at the Land of Memories Park, Sindt ran the boat along three runs. Each run lasts about 20 minutes, or until they have gone about 500 meters.
Before any fishing could begin, Sindt had to take measurements of the river’s temperature and turbidity (water clarity).
Ancillary data such as temperatures and water depth are important because they inform how many fish are caught. As Sindt put it, many factors could influence survey results.
A sudden dropoff in the amount of fish caught during a survey could be due to low temperatures or the river running higher during a survey.
When a river is deeper, it contains more water. The higher water volume means lower fish density so fish are harder to catch.
After collecting some data, Sindt nosed the boat down one bank, keeping the bow and droppers near the land. Doorenbos and Rossow stood out front with nets to grab the many fish that began to float.
The first run was so numerous that Sindt had to stop the boat twice to count large fish and let them go. The live well was filling up.
To count the fish, Sindt grounded the boat on a sand bar or bank. Russow pulled fish from the live well and placed them on a semi-cylindrical measuring board before weighing them and throwing them back.
To count minnows, they separated the small fish by species into smaller tubs. Then each counted and weighed them individually or weighed a portion, say 100. Then they weighed the rest to get a full count with average weights.
The next two runs, one down the opposite bank and one zig-zagging across the center, yielded far fewer fish. The last run caught a shovel-nosed sturgeon which by itself probably raises the Index of Biotic Integrity (IBI) for the river a couple of points, Sindt said.
“Species like blue suckers, shovel nose sturgeon, paddle fish, those are some of the more intolerant species that when you see them, it indicates better water quality and a healthier system,” Sindt said.
The IBI is the system the Minnesota Pollution Control Agency (MPCA) uses to rate the health of the river. The MPCA rates rivers on a scale from zero to 100, the higher the better.
Rivers tend to have far more biological diversity than lakes. The Minnesota River has between 75 and 80 unique species in it.
“They (rivers) have a lot of habitat diversity, a lot of open niches available; a lot of connectivity so the rivers will also get a lot of species that inhabit lakes and streams,” Sindt said.
After an hour of electrofishing, Sindt and his crew caught 883 fish and 25 unique species, including flathead and channel catfish, long nose gar, big and smallmouth buffalo, common carp and sauger. The single largest fish caught was a 17.5 pound flathead catfish.
The data will be sent to the MPCA for analysis later this year. Sindt surveys 16 sites on the river each year, collecting the data necessary to understand the quality of water.
Connor Cummiskey can be emailed at email@example.com.