Discordant Harmonies in Fingernail Clam Populations (Musculium transversum) of Mississippi River Backwater Lakes

The populations of Musculium transversum from 8 backwater lakes have been studied over a 29-year period based upon over 440 grab samples. These lakes in Navigation Pool 9 of the Upper Mississippi River (UMR) 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 summer Muscu!ium 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 m- 2 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