Mussel Counts
Our raw data was analyzed statistically using the computer program, SPSS. Our survey of 19 sites, totaling 76 m2 of the Cannon River, proved to be an insufficient sample size for most methods of statistical analysis. Proptera alata, the most abundant species found in our survey, comprised 42 of 61 total specimens. This prevalence suggests that the species has adapted well to the substrate and feeding conditions in DNR sites, IC and ID.
Our Proptera alata counts show that the species tends to group in locations of cobble sediment. In locations with cobble substrate, we found many more Proptera alata proportionally than we did did in gravel or in sand beds. Our results are consistent with another mussel study (Downing and Downing, 1992) which showed tendencies for mussel to aggregate. Because site selection was necessarily random, we were not able to designate the substrate types surveyed. Sites surveyed were composed chiefly of cobble (see graph). It is therefore difficult to make statistical comparisons of species' substrate preference.
Diversity of samples
The diversity of species found in our survey was lower than expected. In the 76 m2 of the river searched, five different mussel species were found. The most recent Minnesota DNR mussel survey of the area (Davis, 1988) found thirteen different species, indicating greater diversity. This disparity suggests possible local extirpiation since the last survey, but more evidence of this would need to be shown to make any affirmations.
Sediment Release
We estimated there to be 122980 m3 of silt trapped above the Northfield dam. Removal of the dam would presumably release this mass of sediment, affecting fauna downstream.
In accordance with a study of sediment release in the Michigan's Pigeon River (Alexander 1986), we would expect the mussel populations below the dam to decrease. One factor contributing to this decline would be the reduction of certain host fish populations essential to mussels in their glochiodial stage. However, there have been no studies of fish- mussel species interactions in the Cannon. Therefore we can only speculate about the ramifications of fish host extinction. While we did explore the issue by communicating with other researchers who have studied fish host interactions, exacting such relationhips was beyond the scope of this particular study.
Similarly, we would expect population declines in mussels as a result of the physical trauma of such a drastic increase in water turbity. The silt build-up would presumably cause difficulty in mussel filter-feeding (Marking and Bills, 1979). Some mussels species, stuck under the mass of sediment, would be unable to emerge effectively (Marking and Bills, 1979).
Potential Sources of Error
We were unable to collect mussel specimens when heavy rains raised the river's water to dangerous levels. While attempting to collect data under these conditions, we lost control of the boat and both divers had difficulty staying near the dive boat. Some equipment was lost as well. Ideally we would have liked to have surveyed more sites to have a more representative sample of the river. However, weather conditions in July and August made it impossible to survey more sites using our methods.
Possible Future Studies
One way to build upon this project would be to further explore the relationship between mussel species and host fish in the Cannon River. If the dam were to be removed, the entire food chain of the river could be jeopardized by the release of sediment. Therefore, people planning dam removals, development, drainage etc. along the river should carefully analyze species interactions to see how they might be affected by human activities.
When considering the ramifications of removing the Northfield Dam, it is also important to understand how much silt would be released. Further geological studies of the sediment mass would help determine whether dredging the mud out of the river would be beneficial were the dam to be removed. Finally if planners wanted to dredge the area of sediment build-up, they would need a more exact figure of silt volume. While we arrived at a reasonable estimate using a mathematical model, our number was based upon several assumptions about the constancy of the river's shape and depth. Using seismographic equipment, geologists could get a more accurate estimate of silt build-up.
Another means of better ascertaining the status of mussels in the Cannon would be to standardize a search procedure and continue to survey the areas annually. Our results cannot be accurately compared with those of the Davis DNR survey (1988) because different protocols were used. The Department of Natural Resources, the Cannon River Watershed Partnership, and other concerned institutions could get a better idea of the overall health of the river if surveys were conducted in a regular manner.
The Future of Mussels in the Cannon
Of 297 native freshwater Mussels in North America, 213 are listed as endangered or threatened (Williams et. al, 1993). Reasons for extinction include habitat destruction from dams, channel modification, siltation, and introduction of nonindigenous species.
Mussels in the Cannon river have demonstrated considerable resiliance through the age of the pearl rush which decimated populations. They also demonstrate an ability to survive in water of increasing turbidity due to erosion. However, their ability to survive will continue to be tested as erosion continues. It is possible however, that recent changes in construction regulations designed to prevent erosion will improve the water quality for the mussels.
Mussels in the Cannon River may soon have to compete with the exotic species, the Zebra Mussel (Dreissena polymorpha). Introduced to the Great Lakes from eastern Europe, this species is highly competetive and has been known to attach to other mussel species to keep them from feeding. Boaters in and around the Great Lkes have inadvertantly assisted this species' infiltration of the area by transporting mussel larvae in ballast water etc. While our study was being conducted, Zebra Mussels were found in the nearby St. Croix river. This discovery indicates that the Mussels may become part of the rest of Minnesota's waterways soon.
Although the outlook for mussels in the Cannon River may appear bleak, residents of the watershed have begun to take a greater interest in the water quality of the river. Thanks to the Cannon River Watershed Partnership, people are becoming more aware of the issues affecting the river today. Through further education, more people will learn the importance of mussels as indicator species of the overall health of the river. This heightened awareness could ultimately preserve the mussels in the Cannon River.