Dale Hollow National Fish Hatchery
Southeast Region
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Cooperative Freshwater Mussel Propagation/Culture Project

Harvested Mussels

Infesting Host Fish With Glochidia. Credit: USFWS

Flushing Glochidia From Female Mussel 01

Flushing Glochidia From Female Mussel. Credit: USFWS

Freshwater mussels are currently one of the largest faunal groups listed under the Endangered Species Act (ESA). One of the goals of the U.S. Fish and Wildlife Service (USFWS) is to stabilize and recover populations of depleted aquatic species, including mussels, and reduce the need for listing species under the ESA. One might ask, “Why bother? Why worry about an organism that most people never even see?” Freshwater mussels live on the bottom of streams, rivers, and lakes and are an important part of the food web. They are filter feeders, removing algae, bacteria and other organic material from the water, thereby helping to improve water quality. Mussels are, in turn, eaten by predators such as fish, raccoons, and muskrats. These important roles support the cycle of life in the aquatic ecosystems in which these animals occur. Mussels are an important indicator species. They are easily affected by pollution and habitat alteration. Declining mussel populations are an indication that something is not right and corrective action is necessary.

Placing Host Fish In Cages 04

Placing Host Fish In Cages. Credit: USFWS

Corps Boat House 03

Corps Boat House. Credit: USFWS

The Dale Hollow National Fish Hatchery (NFH) took part in a three year mussel culture project, beginning in FY 2000, designed to transfer mussel culture technology from the Tennessee Cooperative Fishery Research Unit to the National Fish Hatchery System (i.e., Dale Hollow NFH, TN and Wolf Creek NFH, KY). This early effort focused on growing juvenile mussels in closed, recirculation systems. The process was initiated by placing mussel glochidia on the gills of host fish held in small aquariums. Glochidia are the parasitic life stage of North American freshwater mussels. They attach to the gills of the host fish and become encapsulated in the gill tissue, deriving nutrients from the fish. Each species of mussel requires a specific species of host fish. The glochidia transformed into juvenile mussels and fell off of the host fish onto the bottom of the aquarium where they were collected. The juvenile mussels were fed algae grown in trout egg hatching jars. As have other mussel holding facilities, Dale Hollow NFH experienced problems keeping juvenile mussels alive for prolonged periods. The problem with growing some species of mussels is being able to meet their nutritional requirements. Developing techniques to enable long term survival and growth is important because the larger we can rear juvenile mussels, the better chance they have to survive when they are released into the wild.

Harvesting Mussels From Cage Bottom 06

Harvesting Mussels From Cage Bottom. Credit: USFWS

Separating Cage From Cage Bottom 05

Separating Cage From Cage Bottom. Credit: USFWS

Culturing mussels in a lab type setting can be very labor intensive and costly. The hatchery became involved in a new project in 2013 to attempt culturing mussels in cages suspended in Dale Hollow Lake in an effort to develop a more effective and efficient method of producing sub-adult mussels for use in ongoing recovery/restoration efforts. Partners included the U.S. Army Corps of Engineers (Corps), Tennessee Cooperative Fishery Research Unit (Unit), USFWS Ecological Services Field Office, Tennessee Wildlife Resources Agency (TWRA), Friends of Dale Hollow NFH (Friends Group), and Natchitoches NFH, Louisiana. The culture methods have evolved over the years but remain basically the same. Mussel host fish are collected from the wild and/or picked up at cooperating hatcheries and are transported back to the hatchery in aerated water containing salt. The fish are then placed into 280-gallon, aerated circular tanks. Lake water is added, as needed, to keep the water temperature at approximately 60 to 65 degrees F. Cooperating malacologists later infest the host fish with mussel glochidia. The infested host fish are then moved from the hatchery to Dale Hollow Lake. The cages are assembled by placing the upper, screen covered sections onto their bases and the cage bottoms (bases) are covered with river sand. Next, divers partially submerge each cage in the lake and host fish are placed into the cages. Cage tops are secured with plastic wire ties and then the cages are lowered into place on the suspended racks. The racks are secured to walkways inside of a Corps boat house located in a small embayment near Dale Hollow Dam. The boat house is an ideal platform for such a project as it is secure, does not require adjustment as the lake level rises and falls, and does not present an obstacle for boat traffic. The host fish remain in the cages for three to four weeks until the glochidia transform into juvenile mussels, detach from the host fish, settle into the sand substrate on the bottom of the cages, and begin filter feeding. Divers then release the host fish back into the lake. This is the point where cage culture differs significantly from earlier laboratory based culture. Earlier trials focused on providing sustenance by feeding juvenile mussels cultured algae whereas this trial relies on the mussels deriving their sustenance by filtering out food items from the lake water. Once the host fish are removed it becomes a waiting game. Water clarity, temperature, and dissolved oxygen are monitored weekly. Juvenile mussels are harvested from the grow-out cages following a 13 to 25 week growing period.

Round Hickorynut Mussels

Round Hickorynut Mussels. Credit: USFWS

Plain Pocketbook Mussels

Plain Pocketbook Mussels. Credit: USFWS

Cage culture trials have taken place every year from 2013 through 2017. A number of different mussel/host fish combinations have been tried, including pink mucket mussel (Lampsilis abrupta)/spotted bass, pink mucket mussel/largemouth bass, pink mucket mussel/walleye, black sandshell mussel (Ligumia recta)/sauger, black sandshell mussel/walleye, and mucket mussel (Actinonaias ligamentina)/spotted bass. There have been three significant modifications to the culture techniques made over the years. First, more effort has been taken to maintain host fish in good health until glochidia infestation and placement of host fish in suspended cages. This entailed more frequent water exchanges and maintaining salt concentrations at 3-5 ppt in the holding tanks. This change was made in order to better alleviate stress and control fungus. The host fish were taken off of the salt treatment just prior to infestation and were placed back on salt one week following infestation. Studies have shown that once the glochidia become encapsulated in the gill tissue of the host fish, they will tolerate some of the chemical treatments used to maintain the health of the host fish. Second, cages were modified by placing false bottoms, constructed out of plastic mesh netting, between the upper section of the cages and sand substrate in order to prevent the host fish from washing substrate and juvenile mussels out of the cages due to fish movement. Third, originally, only floating racks were used to hold suspended cages. We have moved exclusively to stationary racks anchored to walk ways inside of the boat house to hold cages in place. Divers seem to prefer them over the floating racks because they offer a much more stable work platform. Survival rates have varied widely between trials. Although the total number of mussels harvested is not yet consistent, the growth has been phenomenal. Mussels grown in the lake over a fairly short growing season are larger than what one would expect from mussels growing two or three years in a laboratory type setting. In fact, mussels found in the wild that are the same size as those harvested from cages, usually have a number of annuli on the shell which are put down every growing season, whereas cage grown mussels lack growth annuli. All of this makes Dale Hollow Lake look very promising for future, expanded cage culture efforts. All of the mussels produced from early cage culture trials were used to help meet recovery and restoration goals for a Natural Resources Damage Assessment (NRDA) for the Ohio River Islands National Wildlife Refuge. A chemical spill which took place in 1999 severely impacted mussel, snail, and fish populations along a twenty-mile segment of the river.

Placing Newly Formed Plain Pocketbook Mussels Onto Sand Substrate 10

Placing Newly Formed Plain Pocketbook Mussels Onto Sand Substrate. Credit: USFWS

Newly Transformed Plain Pocketbook Mussels 09

Newly Transformed Plain Pocketbook Mussels. Credit: USFWS

A fifth mussel cage culture trial began in the spring of 2017 when personnel from White Sulphur Springs NFH, West Virginia, arrived with gravid, female plain pocketbook (Lampsilis cardium) and round hickorynut (Obovaria subrotunda) mussels and several species of host fish to be used in the trial. Banded sculpin (Cottus carolinae) and fantail darter (Etheostoma flabellare) host fish were collected in east Tennessee with assistance from a TWRA biologist during the trip to Dale Hollow NFH. Thirty sculpin were infested with round hickorynut mussel glochidia and placed in one suspended cage. Thirty-five fantail darters were also infested with round hickorynut mussel glochidia and placed into a separate suspended cage. The 28 surviving largemouth bass host fish being held at the hatchery were infested with plain pocketbook mussel glochidia and placed into 5 suspended cages. High water at the collection site this spring prevented TWRA personnel from being able to collect gravid, female pink mucket (Lampsilis abrupta) mussels so it was decided to use the surviving largemouth bass host fish for plain pocketbook mussel production. A total of 4,121 plain pocketbook mussels and 11 round hickorynut mussels were harvested from cages on October 31, 2017, following a growing period of approximately 25 weeks. The round hickorynut mussel is an at-risk species while the plain pocketbook mussel is a species of special concern in Tennessee. The plain pocketbook mussel is a surrogate for the federally endangered pink mucket. Mussels produced were tagged at White Sulphur Springs NFH, West Virginia, and stocked into a recovering stream in Pennsylvania that had a total aquatic kill caused by a pollution event. A small number of newly transformed plain pocketbook mussels were placed into a suspended cage and left in the lake to analyze potential overwintering survival and growth of juveniles transformed in a lab setting.

Next spring, the focus will be on timing the arrival of host fish more closely with the availability of gravid female mussels and personnel trained in infesting host fish with mussel glochidia, thereby decreasing the amount of time that the host fish have to be maintained in holding tanks. Mussel production depends a lot on the health of the host fish when they are placed into suspended cages. Some species of host fish don’t take handling at the hatchery very well and don’t survive long enough in cages to give glochidia time to transform into juveniles. Every year is a new learning experience requiring constant modification of techniques to increase production.

Last updated: June 7, 2018