LARVAL FISH/ZOOPLANKTON DYNAMICS IN A MOIST SOIL
MANAGEMENT UNIT

Ted E. Snider, James A. Stoeckel, Melissa Goerlitz, and K. Douglas Blodgett 
Illinois Natural History Survey, Long Term Resource Monitoring Program
(LTRMP) La Grange Reach Field Station, 704 N. Schrader Ave., Havana, Illinois
62644 USA

Because larval stages of most freshwater fish species are zooplanktivorous, year-class strength of many species can be strongly dependent upon the availability of
edible zooplankton. Previous studies from systems (mainly Midwestern
reservoirs) into which gizzard shad have been stocked as forage fish for piscivores
have shown that high densities of young-of-year (YOY) gizzard shad can severely
deplete zooplankton resources to the point where the YOY of later-spawning
piscivore species are severely limited by food availability. During 1997 we
examined larval fish and zooplankton dynamics in Wasenza Pool of Lake
Chautauqua, a backwater lake of the Illinois River, to see if the same type of
interactions reported from reservoirs might be occurring in this floodplain lake in
which gizzard shad are a native component of the fish community.  Lake
Chautauqua is currently divided into two pools: Kikunessa and Wasenza.
Wasenza Pool is managed primarily as a moist soil unit for the benefit of
waterfowl and shorebirds. Because this pool is typically flooded from late winter
to early summer, and because the sediments have been compacted by annual
dewatering in summer and fall, it may provide valuable spawning and nursery
habitat for fishes.  From late April through early July, we sampled larval fish via
paired, 10-minute surface tows using 500- m mesh ichthyoplankton nets twice
weekly at up to six randomly chosen sites (3 near shore and 3 offshore) per
sampling date. Depth-integrated zooplankton samples were collected at the same
time and location via a hand operated diaphragm pump and a 55- m plankton
concentrator.  In 1997, the larval fish community was dominated by Clupeidae
(91%) followed by Cyprinidae (3.4%), Catostomidae (1%) and Centrarchidae
(0.7%). Clupeids were collected beginning in mid May whereas Centrarchids were
not collected until early June. Both taxa peaked in mid to late June with
abundances decreasing dramatically by early July. Preliminary data indicate a
zooplankton crash occurred in mid to late June. This zooplankton crash may have
occurred near the time of peak YOY gizzard shad abundance. We are examining
temporal patterns in zooplankton fecundity and larval fish daily ration to evaluate
the relative importance of larval fish predation on fluctuations in zooplankton
abundance and the effect of the zooplankton crash on food availability to larval
fish. In addition, we are investigating whether YOY fish had switched from
zooplankton to alternate food sources (benthic prey, piscivory, seston, etc.) before
Wasenza Pool was drained into Quiver Creek (a tributary of the Illinois River) in
July.  Because zooplankton productivity is typically much lower in rivers than in
lakes, fish produced in managed, backwater lakes may exhibit low survivorship if
they are flushed into the river while they are still dependent on zooplankton as a
primary food source.  

Keywords: larval fish, zooplankton, Illinois River, Lake Chautauqua, floodplain
lake


Ted E. Snider, LTRMP La Grange Station, 704 N. Schrader, Havana, Illinois
62644, phone:309-543-6000, fax:2105, e-mail snider@fgi.net
preference = poster, would rather not convert.