Illinois Natural History Survey - University of Illinois

INHS Reports November-December 1997

Hydroacoustics: A Tool for Understanding
Fish-Habitat Associations in Rivers

Large floodplain rivers, such as the Mississippi and Illinois, are composed of a variety of different habitats (e.g., backwater lake, side channel, main channel). The majority of sampling done to understand the importance of these different habitat types to fish occurs outside the main channel because the main channel has high current velocities and causes most fisheries sampling techniques to be ineffective or too difficult to employ. As a result, present knowledge on fish abundances, species composition, and habitat utilization in the main channel is lacking.

As part of a study on potential effects of commercial navigation of fishes funded by the U.S. Army Corps of Engineers, we are sampling the main channel of the Mississippi and Illinois rivers using a large rockhopper bottom trawl. Our trawl data contradicts existing beliefs that the main channel does not support fishes, instead suggesting that an abundant and diverse fish assemblage exists. Questions such as How do fish survive in high-current velocities that are typical of the main channel? or Where exactly are fish located in the main channel? need to be addressed.

Digital hydroacoustic sampling gear used in the Mississippi and Illinois rivers.

A logical method to address these questions is through the use of hydroacoustics. Hydro-
acoustic techniques were originally developed for deep marine environments by the U.S. Navy. Recently, methods have been refined so that hydroacoustics are effective for sampling fishes in shallower freshwater systems such as reservoirs and rivers.

Using a digital 200-kHz single beam hydroacoustic system, we hoped to investigate these questions. We sampled sites in the upper and lower section of Navigation Reach 26 of the Mississippi River, and the lower section of the Illinois River where it enters Reach 26. We found a distinct difference in bottom formation and hardness when we compared echograms from the Mississippi River to the Illinois River. The Illinois River bottom has little relief and consists primarily of clay, whereas the Mississippi River bottom consists primarily of sand with strong relief. Sand dunes can be created by high-flow rates when the bottom substrate is mainly sand. These dunes (see figure) are a major part of the Mississippi river bottom and may provide an important refuge from high-flow rates for fish.

To assess the importance of these dunes as main channel fish habitat, we recorded the distribution of each individual fish target in 1.0-meter strata of the water column. Dunes were noted when the river bottom depth contour changed more than 0.3 meters within a distance of 10 meters of bottom. Fish target location was then compared to the location of dunes.

Echogram of sand dune with fish targets in the Mississippi River.

Fish targets were strongly associated with sand dunes in the Mississippi River sites. In almost every case when a dune was located, a fish target was found downstream of its peak. As a result, fish distributions were concentrated and very patchy along the river bottom. Fish targets in the Illinois River were more evenly distributed, both vertically and horizontally, than in the Mississippi River.

Sand dunes may be a critical habitat to species that have adapted to the conditions in large- river systems. For example, the shovelnose sturgeon occurs extensively in the main channel of Reach 26 where flow rates are high but has rarely been collected in the lower Illinois River. If sand dunes are an important habitat for native species such as the sturgeon, further research is needed to assess the abundance of this habitat and how recent changes in the river system (e.g., dams) may have an impact on these habitats.

Randy M. Claramunt, John M. Dettmers, David H. Wahl, and Dan Soluk, Center for Aquatic Ecology

Charlie Warwick, editor