IDENTIFYING POTENTIAL CONTROLS ON THE DIVERSITY OF FISHES AND ABUNDANCE AND SIZE 
STRUCTURE OF CENTRARCHIDS IN OFF-CHANNEL AREAS IN THE UPPER MISSISSIPPI RIVER 
SYSTEM: AQUATIC VEGETATION AND PHYSICAL FEATURES

Brent C. Knights, Brian Ickes, Yao Yin and John C. Nelson
U.S. Geological Survey, Upper Midwest Environmental Sciences Center, 2630 Fanta 
Reed Road, La Crosse, WI 54603

      Fish diversity and centrarchid abundance and size structure are of 
significant importance to the public in the Upper Mississippi River System 
(UMRS).  Evidence and perceptions that off-channel areas (OCAs) are important in 
regards to these fishery attributes and that OCAs are degrading has resulted in 
a focus on these areas for restoration.  To date, research on fish diversity and 
centrarchid abundance and size structure have focused almost entirely at the 
Pool scale (38-126 km) because the Long Term Resource Monitoring Program (LTRMP) 
is designed for inferences at that scale.  However, given the OCA-scale 
objectives of many restoration efforts and the hierarchal nature of large river 
ecosystems there is a need for complementary OCA-scale information.  Thus, our 
research sought to determine how OCA-scale patterns in aquatic vegetation and 
physical features are related to centrarchid size structure and fish diversity 
patterns.  We adopted a model-based approach that capitalizes on the full 
history of LTRMP fisheries observations as well as inherent gradients in 
vegetation abundance and physical attributes.  
      The models we developed suggest that the abundance of aquatic vegetation, 
shoreline development, and connectivity were important to fish diversity, small 
centrarchids (bluegill Lepomis macrochirus < 150 mm, black crappie Pomoxis 
nigromaculatus < 200 mm, and largemouth bass Micropterous salmoides < 300 mm), 
and large bluegill (>= 175 mm).  Aquatic vegetation and shoreline development 
were generally positive influences on fish diversity and centrarchid abundance, 
whereas connectivity was a negative influence.  In contrast, physical attributes 
had more prominent importance than vegetation for black crappie >= 200 mm.  For 
largemouth bass >= 300 mm, our models indicated that Pool-scale factors were 
most important in determining abundance patterns.  The proportion of deep water 
(>= 2 m) area was unimportant in predicting fish diversity or centrarchid 
abundance and size structure in OCAs.  In general, fish diversity and 
centrarchid abundance in OCAs should benefit from physical features that produce 
more lentic conditions.  However, literature suggests that some connectivity may 
be necessary to ensure suitable physicochemical conditions in OCAs for fish.  
Aquatic vegetation was likely important for small centrarchids because it 
provides protection from predators and quality habitat for invertebrate prey.  
Vegetation may further benefit large bluegill by partitioning habitat thus 
reducing interspecific competition.  Managers should also consider structural 
complexity and spatial configuration (edge or patchiness) of vegetation beds to 
maximize benefits to centrachids and other fishes.  The unimportance of 
vegetation for large black crappie and largemouth bass may be related to their 
efficiency as piscivores and the abundance of available prey fishes in open 
water habitats in the UMRS.  Our model results, previous Pool-scale research, 
and pertinent literature suggest that restoration planners should consider a 
broad range of physical and floral attributes of OCAs, as well as larger-scale 
factors, to increase fish diversity and centrarchid abundance in the UMRS.
Keywords: centrarchid, fish diversity, off-channel area, vegetation, physical 
feature