MULTIPLE QUANTITATIVE UNIONIDAE SURVEYS OF THE SAME TRANSECT WITH SPECIMEN 
REMOVAL BETWEEN SURVEYS; CHIPPEWA RIVER, WISCONSIN 

Chris Wallace and Terry Balding. University of Wisconsin-Eau Claire, Biology 
Department, Eau Claire, WI 54702

During the summer of 2001 we conducted 4 consecutive unionid surveys along the 
same transect on the Chippewa River, Wisconsin.  We relocated all unionids after 
each survey.  Our purposes in this study were to determine 1) if there was any 
vertical movement by unionids during normal water levels, 2) if smaller 
individuals would be recovered in later swim-over surveys, and 3) if there were 
differences in the sizes or species collected by swim-over versus digging 
surveys.  

From the 4 consecutive surveys of the same transect there were a total of 18 
species found and 542 live unionids were counted (density of approximately 2.8 
unionids/m2).  Among the unionids collected were two Wisconsin Threatened and 
Endangered species, Plethobasis cyphyus and Tritigonia verrucosa.  The dominant 
species were Potamilus alatus and Fusconaia flava, comprising nearly 57.4% of 
all the unionids.

The data from this study provide some indication that within a 46-day period 
unionids may move vertically within the substrate.  The data also show there is 
a significant difference in the mean standardized lengths for the 3 consecutive 
surveys (R2 = 0.9984, 
P ( 0.05).  While there are only 3 data points and the range in size is slight, 
the data indicate swim-over surveys have a bias for larger unionids.  

Overall we found smaller unionids and a greater number during the digging survey 
than the swim-over survey.  Additionally, some species and some sizes were 
under-represented during the swim-over survey compared to the digging survey.  
These findings indicate surveys intending to obtain community and population 
structure should use digging.