DIET COMPOSITION OF LARVAL AND JUVENILE FISH TAXA IN AN ILLINOIS RIVER
FLOODPLAIN LAKE.

A. Maria Lemke, Jim A. Stoeckel, Melissa Goerlitz, Mark A. Pegg.  
Illinois Natural History Survey, Illinois River Biological Station, Havana, IL 62644.

Lake Chautauqua, a floodplain lake on the Illinois River, is managed by the U.S. Fish and
Wildlife Service as a moist soil unit to provide food and refuge for migrating waterfowl and
shorebirds.  Floodplain lakes managed for these purposes are annually flooded with river water
from fall to early summer and subsequently dewatered during the summer.  Additional interest
pertaining to the suitability of Lake Chautauqua as habitat for larval fish has promoted a
multi-year study to investigate the production and subsequent release of larval and juvenile fish
into the Illinois River.  One important component of habitat suitability assessment in freshwater
ecosystems is the extent to which availability of edible plankton affects interspecific competition
among larval fish.  An additional consideration for managed floodplain lakes is how the timing
of dewatering events affects survivorship of larval and juvenile fish released into the river. 
Because zooplankton production is typically lower in rivers than lakes, fish produced in Lake
Chautauqua may exhibit lower survivorship in the river system if they have not yet shifted their
reliance from zooplankton as a primary food resource to fish and/or macroinvertebrates. 

The primary objective of this study was to compare diet compositions among several fish taxa in
order to (1) gain some insight into diet overlap and possible interspecific competition at the
larval and juvenile stages, and (2) determine the size ranges at which piscivorous species shift
their diet from zooplankton to macroinvertebrates and fish.  A secondary aspect of this study was
to investigate the utilization of the exotic claodoceran, Daphnia lumholtzi, by larval and juvenile
fish.  Previous reports from Lake Chautauqua have shown that high abundances of D. lumholtzi
can occur from mid- to late-summer when river water levels exceed levee height of Lake
Chautauqua for extended periods of time.  

Diet compositions were analyzed for 3 larval and 3 juvenile fish taxa collected during 2 separate
escapement periods in early (5-15 June) and late (31 July-2 August) summer, respectively.  The
diet compositions of larval white bass (<20 mm) and freshwater drum (<10 mm) were comprised
almost exclusively of cyclopoid copepods and microcrustacean eggs.  Clupeid larvae (4-24 mm)
fed on a wider variety of small-bodied invertebrates including rotifers, microcrustacean eggs,
cyclopoid and naupliar copepods, cladocerans, and ostracods.  Juvenile white bass (61-142 mm)
and largemouth bass (56-89 mm) exhibited very similar diets comprised of Daphnia lumholtzi,
microcrustacean eggs, corixids, and fish.  Our data indicates that smaller size classes of white
bass (61-100 mm) and largemouth bass (56-65 mm) juveniles fed primarily on zooplankton and
macroinvertebrates, and shifted to a more piscivorous diet after reaching 100 and 72 mm in total
length, respectively.  Daphnia lumholtzi were consumed by several juvenile (white bass,
largemouth bass) and adult (emerald shiner) fish, and comprised a significant proportion (47%)
of juvenile bluegill (41-63 mm) diets.  

Similar diet composition suggests that larval white bass and freshwater drum may compete for
zooplankton during early summer, and that white bass and largemouth bass may compete for
macroinvertebrate and forage fish later in the summer.  Because juvenile centrarchids and white
bass have shifted their diets from plankton to macroinvertebrates and fish by mid- to
late-summer, their survival is likely high in the river system when dewatering occurs later in the
summer.  High abundances of Daphnia lumholtzi in late summer provide an additional food
source for juvenile bass and bluegill when native zooplankton exhibit low abundances.

Keywords:  floodplain lake, Illinois River, larval/juvenile fish, diet composition, Daphnia
lumholtzi