Trawling vessel sampling fishes in the main channel of the Mississippi River.
Large floodplain rivers like the Mississippi and Illinois are dynamic and diverse ecosystems. These rivers are composed of several habitats (e.g., main channel, side channel, floodplain, and backwater lakes) that allow a diverse assemblage of organisms to persist. For instance, the Mississippi River supports over 200 fish species from Minnesota to Louisiana. Because floodplain rivers have many specialized habitats, it is often very difficult for researchers to sample river fishes effectively. As a result, scientists understand little about where some river fishes live, what they eat, and how they interact with other members of the river community.
Perhaps the most difficult habitat to sample is the main channel, where current velocities are highest and extensive commercial navigation occurs. Because researchers historically could not effectively sample the main channel, little is known about how many fishes use this potentially inhospitable habitat. Illinois Natural History Survey (INHS) scientists, in collaboration with the U.S. Geological Survey and the U.S. Army Corps of Engineers, are currently sampling fishes in the main channel of the Mississippi and Illinois rivers near their confluence at Grafton, Illinois, with a specialized trawling vessel. This boat pulls trawls (large nets) along the river bottom to collect fishes living in the main channel. At the end of each net haul, Survey researchers identify, measure, and weigh each fish before returning it to the river.
The fish community of the Mississippi River main channel is more diverse than that of the lower Illinois River near Grafton. During 1996, we collected 24 fish species in the Mississippi River, but only 13 species in the Illinois River. This difference is likely due to the larger size and greater habitat complexity of the Mississippi. Freshwater drum, channel catfish, gizzard shad, smallmouth buffalo, and carp were common in trawl catches in both rivers. Fishes typically thought of as large-river species (e.g., shovelnose sturgeon) were abundant only in the Mississippi. We also caught relatively rare river fishes like lake sturgeon and blue sucker only in the Mississippi River.
Some fish, including shovelnose sturgeon, gizzard shad, channel catfish, and smallmouth buffalo, use the main channel during the entire year; these fishes are suited for life in fast-flowing river conditions. Many other fishes use the main channel only seasonally. Our most diverse catches occur in September and October, when river flows typically are lowest and temperatures are moderate. Under these conditions, fish common in backwaters (e.g., bigmouth buffalo, shortnose gar, and black crappie) can be found in the main channel.
Trawling vessel sampling fishes in the main channel of the Mississippi
River.
Furthermore, fish movements in the main channel may be affected by navigation locks and dams that permit barges to move up and down the Mississippi and Illinois rivers. A lock and dam creates an area for several miles above it that is impounded (creating a more lakelike environment), relative to a free-flowing river. Within each navigation reach, the lower stretch is more lakelike, but the upper stretch more closely resembles a free-flowing, unimpounded river. Our research indicates that some fishes prefer either the free-flowing or impounded areas within a navigation reach for much of the year. For example, blue catfish and bigmouth buffalo occur primarily in the more lakelike part of the main channel. However, species adapted to high river flows (e.g., shovelnose sturgeon, lake sturgeon, and blue sucker) were collected almost exclusively in the more riverine part of the main channel.
By effectively sampling the main channel of large floodplain rivers, we can provide reliable information about the fish community living in this habitat. Fishes occur in the main channel throughout the year, and, especially in the fall, they exhibit high abundance and species diversity. Fish movement and distribution also may be affected by the presence of navigation locks and dams, which provide migration barriers and alter the river for several miles upstream of their location. Continued sampling in the main channel of these diverse ecosystems will allow us to better understand how the entire river functions.
John M. Dettmers, David H. Wahl, and Dan Soluk, Center for Aquatic Ecology
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