CHANGES IN DENSITY AND SIZE DEMOGRAPHY OF THREERIDGE
MUSSELS  (Amblema plicata plicata) IN LAKE PEPIN, MINNESOTA,
AND WISCONSIN

Rick Alan Hart1 and Mike Davis2
1North Dakota State University, Department of Zoology, Fargo, ND
58105.
701/231-7030, rhart@prairie.nodak.edu
2Minnesota Department of Natural Resources, Ecological Services
Section, Lake City, MN 55041. 612/345-3331

Density and size demography of unionid mussel populations were
quantitatively sampled at several mussel beds in Lake Pepin, beginning
in 1990.  Densities of the commercially harvested threeridge mussel,
Amblema plicata plicata (Say, 1817), declined at 5 of the 7 mussel beds
sampled.  The most dramatic decline occurred at the Hok Si La, MN, bed
where average densities of A. p. plicata equaled 21.7/m2 during 1993,
declining to 6.0/m2 in 1995 and 5.0/m2 in 1996 (F=8.820, P=0.0001). 
Densities of non-harvested mussel species increased or remained stable
at all seven beds.  Average shell height of A. p. plicata significantly
decreased at 4 beds, remained stable at 2, and increased at 1.  These
data implicate commercial harvesting as a contributing factor in the
decline of this mussel species.  During this study, zebra mussels,
Dreissena polymorpha, became established in Lake Pepin.  The greatest
density of zebra mussels was found at the King's Coulee bed in 1995
(1750/m2), decreasing to 971/m2 in 1996 (T=2.366, P=0.021).  Although
densities of zebra mussels decreased at this bed, infestations of zebra
mussels upon A. p. plicata did not.  The average number of live zebra
mussels per live A. p. plicata equalled 44 in 1995 and 48 in 1996.  With
the large percentage of infested A. p. plicata at the King's Coulee bed in
1996 (91%), an increase in zebra mussel induced mortality could soon
become evident.  While zebra mussels can not be controlled by resource
agencies, the impacts of commercial harvesting can.  An attempt is being
made to model safe levels of harvesting, thus ensuring a sustainable
yield of A. p. plicata.       

Keywords: Amblema plicata plicata, zebra mussels, Lake Pepin, unionid
mussels, size demography

Rick Hart, North Dakota State University, Department of Zoology,
Stevens Hall, Fargo, North Dakota 58105. (701)231-7030, e-mail
address rhart@prairie.nodak.edu
I prefer to present this research as a platform presentation and I am not
willing to convert to a poster presentation.
The first author of this paper is a Ph.D. student at North Dakota State
University.