TURTLE COMMUNITY STRUCTURE IN A BACKWATER LAKE OF THE ILLINOIS RIVER.

John K. Tucker1 and James T. Lamer2 
1 Great Rivers Field Station, Illinois Natural History Survey, 8450 Montclaire 
Ave, Brighton, IL 62012. 2 Department of Biological Sciences, Western Illinois 
University, 1 University Circle, Macomb, IL 61455.

Turtle communities of large rivers of the United States have received little 
systematic study.  Turtles are important in aquatic environments contributing 
considerable biomass.  Regardless most community studies involve lakes and 
ponds.  We report here on a three year trapping study of Long Lake, a backwater 
lake of the lower Illinois River.  Turtles were trapped each year in July and 
August, which were the only months that water levels and hunting use allowed us 
reliable access to the lake.  Legler hoop traps baited with fish were used in 
each year to catch turtles.  Turtles were identified to species, weighed, 
measured, marked, and released.  Overall we caught 3,473 turtles from six 
species including the spiny softshell (Apalone spinifera), the red-eared slider 
(Trachemys scripta elegans), the painted turtle (Chrysemys picta), common map 
turtle (Graptemys geographica), the stinkpot (Sternotherus odoratus), and the 
common snapping turtle (Chelydra serpentina).  Red-eared sliders dominated 
catches in each year of the study ranging from 90.6% (2002) to 96.8% (2003) of 
the turtles caught.  In contrast, we caught only 2 common snapping turtles in 
Long Lake during the three year study.  Common snapping turtles are usually an 
abundant species and their absence is unexplained.  Sex ratio for various 
species varied from female biased in the spiny softshell (82.7% females) and 
stinkpot (61.7% female) to male biased in the painted turtle (60.7% male).  
Sliders had equal numbers of males and females (50.1% male).  Male sliders 
averaged 146 mm in plastron length and 675 g in mass.  Female sliders averaged 
171 mm in plastron length and 1098 g in mass.  Catch per unit effort increased 
dramatically from 2001 (3.43 turtles/trap/day) to 2003 (20.88 turtles/trap/day).  
The change appears to be due to increased numbers of small turtles caught in 
2003.  Density can be estimated for the entire lake at about 171 
sliders/hectare.  However, a more realistic estimate of density based on the 
actual area trapped is 433 sliders/hectare.  The slider density (433 
turtles/hectare) for the actual area trapped is similar to the number of sliders 
per hectare reported for stream and pond habitats elsewhere.  Estimated biomass 
for sliders was a minimum of 128 kg/hectare for the entire lake and about 325 
kg/hectare for the area actually trapped.  These estimates are comparable to 
biomass estimates for fish communities suggesting that turtles are an important 
but largely ignored component of the ecosystem in river backwaters.  

Keywords: Illinois River, turtles, turtle biomass, population increases, 
community structure.