ZEBRA MUSSEL VELIGER SUPPLY AND SETTLEMENT IN THE ILLINOIS RIVER

Angela Nealand 1, James A. Stoeckel 1, and Daniel W. Schneider 2.
1 Center for Aquatic Ecology, Illinois Natural History Survey, Long Term Resource 
Monitoring Program, Havana Field Station, 704 North Schrader Avenue, Havana IL 
62644; and 2 Department of Urban and Regional Planning and Illinois Natural History 
Survey, University of Illinois at Urbana-Champaign, Champaign, IL 61820.

Zebra mussels (Dreissena polymorpha) will probably never be completely eliminated 
from U.S. waterways, but there is hope that their population may be reduced in river 
systems.  Zebra mussels in the Illinois River likely function as a metapopulation (a 
system of local populations connected by dispersal).  Adult populations are dependent 
upon an upstream veliger supply since internal recruitment is limited by river currents. 
Because of this dependence, zebra mussels might be controlled by preventing the 
influx of veligers.  An important question to investigate is how tightly this influx is 
linked to settlement.

In order to explore this link, we monitored larval supply and settlement rates during 
1999 at two fixed sites on the Illinois River near Havana (river miles 115 and 121.1). 
Larval supply was estimated by means of 30 liter samples collected 0.25 m off the 
bottom of the river.  Settlement was monitored on both a weekly and a monthly basis 
using racks of ten three-inch diameter clear PVC plates.  Post-settlement survivorship 
and growth was measured using a photoplate sampler comprised of a rack of eight 
six-inch square PVC plates.  These plates were photographed and returned to the 
water throughout the season.  Natural communities were allowed to develop on these 
plates to limit disturbances and allow normal settlement.  The three methods could 
then be compared to find the most accurate measure of settlement.

The only major settlement event occurred during a three week period from mid May 
to mid June.  Preliminary results indicate that the different sampling methods show 
different attachment rates and that settlement is highly variable between replicates. 
Higher than normal levels of umbonal veligers were present in zooplankton samples 
during the settlement event.  We are currently investigating the size composition of 
the larval flux, evaluating settlement on the photoplate samplers, and analyzing 
weekly and monthly settlement results.

Keywords:  zebra mussel, Illinois River Dreissena polymorpha, metapopulation
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