Background: Freshwater mussels remain among the most imperiled species in North America due primarily to habitat loss or degradation. Understanding how mussels respond to habitat changes can improve conservation efforts. Mussels deposit rings in their shell in which age and growth information can be read, and thus used to evaluate how mussels respond to changes in habitat. However, discrepancies between methodological approaches to obtain life history information from growth rings has led to considerable uncertainty regarding the life history characteristics of many mussel species. In this study we compared two processing methods, internal and external ring examination, to obtain age and growth information of two populations of mussels in the St. Croix River, MN, and evaluated how mussel growth responded to changes in the operation of a hydroelectric dam.Results: External ring counts consistently underestimated internal ring counts by 4 years. Despite this difference, internal and external growth patterns were consistent. In 2000, the hydroelectric dam switched from operating on a peaking schedule to run-of-the-river/partial peaking. Growth patterns between an upstream and downstream site of the dam were similar both before and after the change in operation. At the downstream site, however, older mussels had higher growth rates after the change in operation than the same sized mussels collected before the change.Conclusions: Because growth patterns between internal and external processing methods were consistent, we suggest that external processing is an effective method to obtain growth information despite providing inaccurate age information. External processing is advantageous over internal processing due to its non-destructive nature. Applying this information to analyze the influence of the operation change in the hydroelectric dam, we suggest that changing to run-of-the-river/partial peaking operation has benefited the growth of older mussels below the dam.
Bibliographical noteFunding Information:
This work was supported in part by the Undergraduate Science Education Program Grant No. 52006323 from the Howard Hughes Medical Institute to Washington & Jefferson College, a Margrett A. Cargill Foundation restricted grant to the Environmental Studies Program at Washington & Jefferson College, National Park Service cooperative agreements PMIS 111881 and PMIS 131196, and Macalester College faculty travel fund. We thank the following people for help collecting mussels, and preparing and interpreting thin sections: Derek Ochi, Elise Griffin, Karen Jackson, Carl Skarbek, Kelly MacGregor, Jeff Thole, Ronald Bayline, Jamie March, and Robert East. We wish to thank both biology departments at Macalester College and Washington & Jefferson College who helped facilitate this collaborative research. Finally, thanks to two anonymous reviewers for helpful comments on the manuscript.
- Flow regulation
- Growth processing methods
- Hydroelectric dam