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Hart, R.A. 1999. Population dynamics of unionid mussels in Lake Pepin, Upper Mississippi River, Minnesota and Wisconsin. Ph.D. Dissertation, North Dakota State University. 162 pp.


To assess the effects of commercial harvest on A. plicata (Say 1817) populations and colonization by D. polymorpha (Pallas 1771), population densities and survival of unionid mussels were estimated. From 1990-97, 7 mussel beds were quantitatively sampled to determine mussel densities. Additionally, to measure survival rates, 240 A. plicata mussels were marked in each of 3 beds in Lake Pepin. The 3 beds were chosen based on known D. polymorpha densities, with each bed having a density of, 0-10, 25-100, or >250 D. polymorpha/m2. For a reference sample, 240 A. plicata were marked where no D. polymorpha were present in the Otter Tail River, MN. Also, 79 D. polymorpha colonized Fusconaia flava (Rafinesque 1820) and 240 colonized Elliptio dilatata (Rafinesque 1820) mussels were marked in Lake Pepin, while 240 F. flava and 113 Lasmigona costata (Rafinesque 1820) were marked in the Otter Tail River. Densities of A. plicata showed significant declines (p<0.05) at 5 of the 7 mussel beds sampled. Densities of non-harvested mussels remained constant in all but one bed. We attribute these declines to commercial harvesting. Mean annual survival of marked A. plicata from 1996-98 was significantly greater at beds with low densities or absence of D. polymorpha (98%, 99%, and 98%) compared to those heavily colonized (76%) (?2df = 3 =93.93, p<0.0001). Survival rates of Otter Tail River L. costata and F. flava were greater than 99%, while survival of E. dilatata and F. flava from Lake Pepin equaled 95% and 90%, respectively. Differences in survivorship of A. plicata and F. flava were attributed to the high densities of D. polymorpha. These survival rates were used in simulations of A. plicata populations under varying levels of harvest and D. polymorpha colonization. Sensitivity analysis revealed population growth to be most sensitive to alterations in adult survival. Simulations predict that the current levels of mortality attributed to D. polymorpha may result in local population extirpations in less than 50 years. Models simulating harvesting revealed that a 5% take of adults may result in population decline of 50% in less than 40 years. This research provided estimates of mean annual survivorship of several mussel species and made new and important contributions to the study of freshwater mussel life-history and population dynamics.

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