HYBRIDIZATION BETWEEN SILVER AND BIGHEAD CARP IN THE MISSISSIPPI AND ILLINOIS 
RIVERS

James T. Lamer1, Chad R. Dolan1, John H. Chick1, and John M. Epifanio2

1Illinois Natural History Survey, Great Rivers Field Station, 8450 Montclair 
Ave., Brighton, IL  62012  

2Illinois Natural History Survey, Center for Aquatic Ecology and Conservation, 
1816 S. Oak St., Champaign, IL  61820
      Bighead and silver carp can be differentiated from one another by a number 
of distinct morphological characteristics.  Fish sharing morphological 
characteristics of both bighead and silver carp have been captured in Swan Lake, 
a contiguous backwater lake located on river miles 5-12 along the Illinois River 
in Calhoun County, Illinois and at 3 locations on Pool 26 of the Mississippi 
River.  A small biopsy of muscle, liver and eye were collected from these 
morphological intergrades and compared to field-identified bigheads and silvers 
at several enzymatic loci via starch gel allozyme electrophoresis.  In addition 
to the morphological variants showing a high degree of genetic hybridization, 
some of the fish identified in the field as true bigheads and silvers also 
displayed hybrid genotypes, with few fish exhibiting pure bighead or silver 
genotypes.  These data suggest a potential for a hybrid swarm between these two 
species.  Replacement of pure bighead and silver carp by some degree of bighead 
X silver hybrid is a plausible concern.  The impact of this hybrid condition, 
both ecologically and taxonomically, warrants further attention.  Additional 
collections and analyses of fish from alternate sites along the Mississippi and 
Illinois Rivers are being conducted.  The use of microsatellite and mtDNA 
markers will be implemented in future investigations.
Keywords:  Asian carp, hybridization, Mississippi and Illinois Rivers, hybrid 
swarm, ecological impacts