Radio Tracking Northern Pike
Posted: 02/21/2012
As Royce Bowman shuffled down the ice-covered backwater, we held back and watched. In one hand, Bowman held out a small antenna. Cradled in his other arm was a radio receiver.

“Even footsteps on the ice can push that fish out of there,” cautioned Denny Weiss, fisheries technician from the Department of Natural Resources. “If we were along, that northern would be long gone.”

We were lagging behind, ready to come roaring up in an airboat when Bowman signaled us. There was a lot of ice—and water—to cover as the two fisheries technicians from the research side of the DNR’s Bellevue station track down northern pike along the Mississippi River late this winter.  The ice was nine inches thick on this day, in the Sny Magill bottoms, north of Guttenberg. Still, there was open water in places. The air-inflated, fan-driven boat was the only way to fly.

Specifically, Bowman was trailing #48.195, a 26-inch, four-pound northern, caught and released November 21, not far from here. It was one of 40 implanted with shotgun shell sized radio transmitters; in the pools above Guttenberg and Bellevue.  Another 680 fish each carry a much less obtrusive ‘floy’ tag, to identify it if recaptured by fisheries workers or caught by anglers in the next three or four years.

“By following these fish year round, we will learn what kind of habitats are important; especially during the winter,” explained Weiss. “A lot of our backwaters are filling in with sediment. If we spend money to dredge some, we want to make sure we do it in the right locations.”

Each pool—upstream from each lock and dam—impounds dozens of shallow backwater lakes, off the main channel. The best ones hold large quantities of bluegills, crappies and other panfish…as well as predators like northern pike. All seek the slower moving water, with adequate oxygen, to make it through winter. Over time, as those backwaters fill in, the habitat deteriorates. Dredging restores them, but at a million bucks a pop--or more--it can’t be done in too many locations.

Catching up with Bowman now, it was time to get the readings. From here, we didn’t need #48.195 anymore. “It took off, as soon as it heard the boat coming,” said Bowman. But he had locked in on the location of the fish…before the disturbance.

“This antenna is directional. Once we locate a fish, we come at it from different angles,” explained Bowman. “When we get right on top of it, we drill a hole and collect data; GPS coordinates, ice thickness, depth of the water below the ice, oxygen readings; even water clarity.”

This four year study looks at distribution of the fish; also age, growth, reproduction and basically their overall abundance.

“We will follow them through warm weather, too,” emphasizes Weiss. “We know in July and August, as the river gets low and the temperatures rise, these northern pike seek out coldwater streams and springs that empty into the Mississippi. We are near the southern edge of the fish’s range. Those cold water refuges might be very important.”

Northern pike are also important many miles from their Upper Mississippi ‘home base.’ Each year as the ice goes out, wild, captured northerns are stripped of eggs and spawned at the DNR’s Guttenberg and Spirit Lake hatcheries. Hundreds of thousands of fry are packed up and shipped across Iowa; to be stocked. Tens of thousands of others are raised to fingerling size and stocked later in the spring.

Many are there for the angler. Most, though, help balance (that is; they eat) populations of game fish; to ensure that no one type gets too large; stunting size structure or availability of others.