Monitoring fitness of caged mussels (Elliptio complanata) to assess and prioritize streams for restoration in Southeastern Pennsylvania

Matthew W. Gray

doi:10.17918/etd-3185

Freshwater mussels (Order: Unionidae) are currently the most imperiled organisms in North America. Restoration of these animals is becoming an increasing research interest for conservationists and resource managers. Historically, restoration methods have yielded less than optimal survival rates due in part to an inability to identify suitable habitat for these organisms. Through the use of caged mussels as bioindicators, a method was developed to identify suitable habitat for supporting mussel fitness prior to beginning actual restoration efforts. To assess the effectiveness of Elliptio complanata to serve as a bioindicator of stream suitability, its physiological plasticity was also examined. Mussels were caged and reciprocally transferred between two source streams in Southeastern Pennsylvania (Brandywine Creek and Ridley Creek). Mussel condition index and proximate biochemical composition (protein, carbohydrate, lipid) were monitored seasonally and related to expected seasonal nutritional demands for the different biochemical constituents. Controls consisted of caged mussels from source streams that were deployed back into their source streams, and the physiological response to caging was determined in contrast to that of uncaged mussels from the same source beds. No caging effects were observed. The physiology of this species was found to be quite plastic, showing rapid adaptation to varying environmental conditions in the new streams. Additionally, the seasonal variation in condition index was similar in some aspects to that of marine species, but did not always follow the expected pattern. To test candidate restoration streams for suitability, mussels from Brandywine Creek were deployed in five candidate streams for restoration. Four of the five streams supported the same or even greater mussel fitness than in Brandywine Creek, which was the main source stream that still harbors extant populations of Elliptio complanata, suggesting that those streams (Chester, White Clay East Branch, White Clay Middle Branch, Red Clay) are suitable for restoration; whereas, West Branch Brandywine Creek was not found to be suitable for mussels. The proximate biochemical composition of stream seston (i.e. microparticulate food for filter-feeders) was also analyzed to partially explain disparities in stream suitability for mussels. Hydrological and water chemistry data were also collected and contrasted among source and indicator streams. Food quality varied widely among studied streams, suggesting that seston biochemical composition was likely to have affected in the suitability of these streams for sustaining healthy mussels. Candidate restoration streams were ranked for their suitability. Since the Ridley Creek source population was significantly more fit than the Brandywine population, I recommend that this population be targeted for special protection and possible enhancement since it appears to serve as the best healthy reference population within the study area. Although mussels still exist in Brandywine Creek, there are indications that the population is not naturally reproducing and this mussel bed may benefit from enhancement and restoration. Mussels have been extirpated from Chester and White Clay drainages, but water quality and food conditions appear suitable there for sustaining mussels that could be restored. Middle Branch White Clay, East Branch White Clay and Chester Creek were found to be similar or greater in suitability than Brandywine Creek. In particular, Middle Branch White Clay was found to have the greatest food quality of any candidate stream and mussels held in this stream were found to be more fit than that of any other candidate stream. At this time, restoration is not prioritized for Red Clay and West Branch Brandywine due to inadequate water quality and habitat conditions for mussels. Since mussels are long-lived and excellent bioindicators of stream conditions, this caging methodology could be used for bioassessment as well as for gauging stream readiness for mussel restoration.

Monitoring fitness of caged mussels (Elliptio complanata) to assess and prioritize streams for restoration in Southeastern Pennsylvania

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Monitoring fitness of caged mussels (Elliptio complanata) to assess and prioritize streams for restoration in Southeastern Pennsylvania

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