SIMPLE STOCHASTIC MODELS TO EXPLAIN POPULATIONS OF THE FINGERNAIL CLAM, 
MUSCULIUM TRANSVERSUM.

Jim Eckblad,  Department of Biology, Luther College, Decorah, Iowa 52101

The populations of Musculium transversum from 8 backwater lakes have been 
studied over a 29-year period based upon over 432 grab samples.  These lakes in 
Pool 9 of the Upper Mississippi River range in size from 2 to 255 hectare 
surface area, mean depths from 0.2 to 0.9 meters, with mean water exchange times 
from  0.5 to almost 57 hours.  Samples from the 1989-91 period suggested that 
summer Musculium populations had declined to about 9 percent of their mid-1970 
levels.  Deterministic models have attempted to explain this decline.  More 
recent sampling suggests a substantial recovery of Musculium populations (mid-
summer mean of 560 per square meter in 2002).   Simple stochastic models, with 
realistic levels of POISSON and NORMAL variables, may provide a better 
explanation of population fluctuations than previously used deterministic 
models.

Keywords:  fingernail clam, Musculium transversum, Mississippi River,  
stochastic models