FISH POPULATION DYNAMICS OF ANNUALLY-FLOODED SEASONALLY-ISOLATED BACKWATER 
LAKE OF THE ILLINOIS RIVER

Matt Stroub and Greg G. Sass

Illinois River Biological Station, Illinois Natural History Survey, 704 N. 
Schrader Avenue, Havana, Illinois 62644

Connectivity and isolation from rivers may be major drivers of fish community 
composition in backwater lakes.  Matanzas Lake is a seasonally-isolated 
backwater lake of the La Grange reach of the Illinois River.  Matanzas Lake 
was flooded and connected to the Illinois River from late February to mid-
May, 2007. From mid-May to present, Matanzas Lake has been isolated from the 
Illinois River.  Weekly, we used a multiple-gear approach to sample the fish 
community of Matanzas Lake both pre- and post-isolation from the Illinois 
River. We used single and tandem fyke nets, trammel nets, experimental gill 
nets, and day electrofishing at fixed sites to assess the fish community of 
Matanzas Lake over time.  All fish captured were identified to species, 
measured, and weighed. Basic water quality information (e.g. water 
temperature, dissolved oxygen) was also collected at all fixed sites on each 
sampling date.  Our objectives were threefold: 1) to test for differences in 
fish species community composition and catch rates from Matanzas Lake pre- 
and post-isolation from the Illinois River; 2) to test for differences in 
fish species diversity among the La Grange Reach of the Illinois River and 
Matanzas Lake; and 3) to identify unique fish species to Matanzas Lake. 
Electrofishing catch rates of the ten most common fishes, except emerald 
shiner Notropis atherinoides Rafinesque, were greater during isolation from 
the Illinois River. Eight of ten fish species showed statistically greater 
electrofishing catch rates. Fish species diversity of Matanzas Lake in 2007 
was greater than the 17 year LTRMP fish species diversity average of the La 
Grange reach of the Illinois River. There was also six species of fish that 
we considered unique to Matanzas Lake, orange spotted sunfish Lepomis 
humilis, grass pickerel Esox americanus vermiculatus, mooneye Hiodon 
tergisus, Yellow Bass Morone mississippiensis, red shiner Notropis lutrensis 
and green sunfish x bluegill. We are uncertain if conductivity to the main 
stem river has affect on the fish diversity and fish community of these 
important backwater lakes.