Field Science—Biologists at the Iowa Department of Natural Resources tracked crappies and bluegills in pools along the Iowa portion of the Mississippi River from winters of 1998-1999 to 2008-2009.* The goal was to identify wintering habitat to help guide management of habitat in rivers. Management on the river. Mike Steuck and Kirk Hansen say that their research stems from previous findings by Iowa biologist John Pitlo, who observed largemouth bass returning to the same backwaters during winter each year.
Biologists implanted radio transmitters in crappies and bluegills in fall and followed their movements over winter, until transmitter batteries expired, about 6 months. "In summer, crappies and bluegills were scattered and used a variety of habitats such as side channels and other areas off the main navigation channel," Steuck says. "As fall arrived, they began to stage closer to the entrances to backwaters. By ice-over, almost all tagged fish were found in backwaters.
"Water temperatures falling below about 50°F cues movements into backwaters. Backwaters used as wintering habitat can be as small as 1/2 acre. The best ones have ample dissolved oxygen, no current, and water temperatures 2°F to 5°F warmer than the main river. Backwaters at least 6 feet deep tend to be used more, as deeper water in ice-covered backwaters allows slightly warmer water near bottom, up to about 39°F.
"Locating structure and cover is a key to finding backwater crappies in winter. They are cover oriented, with an affinity for snags, weeds, docks, and pilings, depending on what's available. In one backwater they related to weedlines, in another marina areas, or you might find them around lotus and lily stems."
*Steuck, M. J. 2010. An evaluation of winter habitats used by bluegill, black crappie and white crappie in Pools 11-14 of the Upper Mississippi River. Iowa Department of Natural Resources, Federal Aid in Sport Fish Restoration, Project F-160-R, Study 7021 Completion Report, Des Moines, Iowa.