Big South Fork restores endangered mussels
By Steve Bakaletz and Dick Neves

“Artificial propagation can dramatically increase the successful recruitment of juveniles into the populations.”

THE RECOVERY OF FIVE FEDERALLY ENDANGERED mussel species in the Big South Fork of the Cumberland River in north-central Tennessee and southeastern Kentucky is being implemented through the propagation of juveniles. An environmental assessment for recovery of these mussel species was completed in fall 2003, endorsing augmentation and reintroduction of federally listed species as the preferred conservation action. The river is currently home to 26 mussel species, including the endangered Cumberland elktoe (Alasmidonta atropurpurea), Cumberland combshell (Epioblasma brevidens), Cumberland bean (Villosa trabalis), tan riffleshell (Epioblasma f. walkeri), and littlewing pearlymussel (Pegias fabula). Because of damming and other impacts, the Big South Fork remains the last stronghold for these species within the Cumberland River system, and is of national significance to the conservation of mussel resources in the United States.

A cadre of partners representing federal agencies and state natural resource departments teamed up with the Freshwater Mollusk Conservation Center at Virginia Tech to augment resident endangered mussel populations and to plan for the restoration of species now extirpated from the park. Historical collection records document 55 mussel species that once resided in the river, including 4 endangered species: clubshell (Pleurobema clava), cracking pearlymussel (Hemistena lata), dromedary pearlymussel (Dromus dromas), and orangefoot pimpleback (Plethobasus cooperianus). This conservation project is consistent with the 1973 Endangered Species Act, the goals of the 1916 National Park Service Organic Act, and the national recovery plans for each of these species.

A preliminary research and feasibility study in 2002–2003 provided essential information on host fish requirements for the parasitic larvae of these species, and allowed juveniles to be experimentally produced and cultured in recirculating culture systems at Virginia Tech, to monitor growth and survival of the early life history stage.

Jess Jones, Virginia Tech

Jess Jones, Virginia Tech

Freshwater mussel restoration at Big South Fork involves several partners (top) working to optimize laboratory propagation methods of endangered mussel species, followed by the release of juveniles in the park. In 2004, approximately 12,000 mussels, including the littlewing pearlymussel (bottom), were introduced to park waters.

Although each female may contain between 1,000 and 100,000 larvae for release to the river and attachment to host fish, research results suggest that fewer than 1% of the larvae attach and transform to the juvenile stage under natural conditions. Hence, artificial propagation can dramatically increase the successful recruitment of juveniles into the populations. During this exploratory stage of the project, more than 42,000 juveniles of four of the endangered species were released to an extensive shoal area of the river, to augment natural reproduction at this site. The release of juveniles two weeks to six months of age bolstered recruitment of naturally produced juveniles in the river.

The laboratory propagation process begins with collection of egg-carrying females from the river. Larvae are removed without harm to the mussels and are induced to attach to the gills of suitable host fish. Female mussels are returned to the river unharmed, to reproduce in subsequent years. The infested host fish are held in tanks at controlled temperatures until the larvae transform from the parasitic larval stage to the free-living juvenile stage. These juveniles are then placed in water recirculating systems with a layer of fine sediment and daily additions of small unicellular algae as their main diet. Once juveniles achieve a size large enough to avoid being consumed by most invertebrate predators in their natal rivers, they are released to fend for themselves and grow to adulthood.

“Approximately 12,000 juveniles were released to the river upon completion of the culture experiments.”

In FY 2004, this restoration project focused on four of the five endangered species with previously identified host fish. Experiments conducted with juveniles of the Cumberland combshell and littlewing pearlymussel tested various culture conditions to improve their survival within the recirculating aquaculture systems. Survival success can differ greatly among broods, and the causes for this variability continue to be evaluated through the manipulation of culture conditions. During this period, approximately 12,000 juveniles were released to the river upon completion of the culture experiments. Plans for 2005 include additional host fish testing and the production and release of additional endangered juvenile mussels at sites selected by park personnel.

It will take several years to evaluate the success of this recovery project, but the documented success of such releases in other rivers provides confidence that endangered populations of resident species and perhaps extirpated species will one day thrive in this national park unit.