FISH ASSEMBLAGES IN OFF-CHANNEL AREAS OF THE UPPER MISSISSIPPI RIVER SYSTEM

Brent C. Knights1, Brian S. Ickes1, Jeffrey N. Houser1, and Melvin C. Bowler2

1U.S. Geological Survey, Upper Midwest Environmental Sciences Center, 2630 
Fanta Reed Road, La Crosse, WI 54603; 2Iowa Department of Natural Resources, 
206 Rose Street, Bellevue, IA, 52031

Off-channel areas (OCAs) are central to the productivity and diversity of 
large floodplain river systems like the Upper Mississippi River System 
(UMRS).  Habitat enhancement efforts focus on OCAs because of their 
ecological and recreational importance and degrading condition.  System 
changes to accommodate navigation and agriculture in the floodplain and 
watershed of the UMRS have altered the physical, biogeochemical, and 
biological regimes of OCAs.  These altered regimes are the new environmental 
template that regulates UMRS fish assemblages.  
   Past research on fish communities in the UMRS has primarily focused at the 
navigation pool scale and homogenized responses at smaller spatial scales.  
We hypothesized that 1) important sources of heterogeneity in select indices 
of physical and biological features exist at smaller scales within the UMRS, 
and 2) spatial patterns in select sub-assemblages of UMRS fish communities 
can be explained by spatial differences in environmental conditions in OCAs.  
We modeled existing UMRS observational data from the Long Term Resource 
Monitoring Program to describe fish assemblage associations with 
environmental conditions at OCA scales.
   Six “major” fish assemblage types were identified amongst 35 OCAs studied 
from Pools 4, 8, and 13 in the Upper Mississippi River and La Grange Pool in 
the Illinois River.  As with previous studies, spatial patterns of assemblage 
type were evident at the pool scale.  Only one of six major assemblages was 
represented in OCAs from more than one pool.  However, multiple assemblages 
occurred in a single pool and some of those were more similar to assemblages 
in other pools than to assemblages in the same pool; this pattern was 
primarily explained by major differences in assemblage between OCAs in upper 
and lower portions of some pools. Fish assemblages ranged from those 
typically associated with degraded conditions dominated by common carp and 
freshwater drum (i.e., OCAs in upper Pool 4) to those reflective of a good 
recreational fishery dominated by centrarchids (i.e., OCAs in lower and 
middle Pool 8 and 13).  The assemblage in OCAs of the La Grange Pool was 
dominated by fishes characteristic of turbid reservoirs including common 
carp, smallmouth buffalo, black and white crappies, white bass, freshwater 
drum, and gizzard shad.  The environmental variables most closely related to 
assemblage types in OCAs included total suspended solids, proportion of 
surface area greater than 1 meter deep, and total nitrogen concentration.
   This research suggests that environmental conditions at scales smaller 
than pool influence fish assemblages.  Further it suggests that OCAs in upper 
Pool 4 may be the most degraded of those examined, possibly caused by high 
turbidity, sedimentation, and nutrients.  The fish assemblage can be viewed 
as a manifestation of a broad array of physical and biological processes in 
an OCA, and as such, a good indicator of system condition under a process-
based management approach as proposed by a regional science panel in the 
UMRS.

Keywords: fish assemblage, off-channel area, environmental factors, Upper 
Mississippi River System

1. Presenter:  Brent C. Knights, USGS_UMESC, 2630 Fanta Reed Road, La Crosse, 
WI, 54603
      608 781-6332, bknights@usgs.gov 
2. Format: Platform.  Not willing to convert. PowerPoint.