Unionid mussels were collected by quadrat sampling at three mussel beds in Reach 15 of
the Upper Mississippi River (UMR) between July 1994 and September 1995. A total of
7,107 unionids were collected representing 26 species, including the federally
endangered Lampsilis higginsi, state endangered Plethobasus cyphyus and Cumberlandia
monodonta, and state threatened Ellipsaria lineolata. Mean densities at our study sites
ranged from 53.4 to 118.3 mussels/m2. Comparisons with data collected at these same
sites in the early 1980s revealed significant declines in unionid abundance, sporadic
recruitment, and extremely slow growth rates. Height-frequency histograms for
commercially harvested species remain truncated at the legal harvest size within the
mussel refuge, suggesting illegal harvest. Zebra mussels (Dreissena polymorpha) have
become well established in Reach 15 with mean densities of 2,519 ZBM/m2 in July 1995
and unionid infestation at 48% by September 1995. Our data analysis and observations
support the following management actions: (1) closing the commercial harvest of live
Megalonaias nervosa in the UMR, (2) establishing entire reaches of the UMR as mussel
refuges, (3) developing population models to guide and assist the management of
mussels, and (4) monitoring zebra mussel densities and impacts in the Mississippi River.
