REDUCED CONDITION FACTOR OF TWO NATIVE FISH SPECIES COINCIDENT WITH INVASION OF 
NON-NATIVE ASIAN CARP IN THE ILLINOIS RIVER: EVIDENCE FOR COMPETITION AND 
REDUCED FITNESS? 

Kevin S. Irons1, Greg G. Sass1, Michael A. McClelland1, and Joshua D. Stafford2

1Illinois River Biological Station, Illinois Natural History Survey, 704. N. 
Schrader Ave., Havana, Illinois 62644, USA

2Forbes Biological Station, Illinois Natural History Survey, P.O. Box 590, 
Havana, Illinois 62644, USA


      Non-native, Asian bighead Hypophthalmichthys nobilis and silver carp H. 
molitrix, have been present in the Illinois River since the early 1990’s.  The 
U.S. Army Corps of Engineers’ Long Term Resource Monitoring Program (LTRMP) has 
collected bighead and silver carp during routine monitoring of the La Grange 
Reach, Illinois River since 1995 and 1998, respectively.  Annual recruitment of 
bighead and silver carp has been variable, but carp biomass in the La Grange 
Reach has increased exponentially since 2000 and these exotics now dominate the 
fish community.  Previous research suggests that dietary overlap exists between 
both Asian carp species and two native Illinois River fishes; gizzard shad 
Dorosoma cepedianum and bigmouth buffalo Ictiobus cyprinellus.  We used length 
and weight data collected during 2000-2005 from over 2,400 fish to evaluate body 
condition of shad and buffalo since Asian carp establishment.  We evaluated body 
condition to assess potential competitive interactions and consequences to 
fitness due to these exotic species.  We identified significant declines in body 
condition of shad and buffalo following Asian carp invasion.  We observed 
statistically significant decreases in shad condition between 2002 and 2005 (p < 
0.001), while buffalo condition decreased between 2000 and all subsequent years 
analyzed (p < 0.003).  To our knowledge, the LTRMP provides the first 
correlative evidence of direct negative competitive interactions between bighead 
and silver carp and native fish species.  We caution that reductions in shad and 
buffalo body condition may lead to reduced fitness, thereby potentially 
accelerating biotic homogenization of Asian carp in the Illinois River and 
causing further disruptions of the native food web and fish community.    

Keywords:  bighead carp, bigmouth buffalo, body condition, gizzard shad, 
Illinois River, LTRMP, non-native, silver carp