Researchers perfect freshwater mussel propagation techniques
By Su Clauson-Wicker
Mussels function as the “good guy” organisms of the aquatic environment. In the process of getting their daily nutrition, mussels filter suspended particles, organic matter, and pollutants from the water in which they live.
Large mussels can filter up to 50 gallons of water a day, making them one of nature’s most effective ways of restoring life to rivers that are no longer pristine. They improve stream life by retaining carbon, controlling the growth of algae, and excreting nutrients for other organisms.
But mussels are becoming increasingly endangered around the world, even in Southwest Virginia and east Tennessee, a region where more than 70 species of freshwater mussels have lived and flourished for millennia.
Livestock wastes, urban discharges, industrial spills, road runoff, and mining have affected the region’s mussel populations.
“Over the last 35 years, there has been a precipitous decline in the diversity and abundance of freshwater mussels,” said Richard Neves, a professor emeritus of fisheries in the College of Natural Resources and Environment and a pioneer in the field of freshwater mussel ecology and propagation. “Of at least 45 species of mussels in the region’s rivers, 24 are listed as federally endangered.”
Federal and state agencies and other groups are working to clean up the region’s rivers and restore native mussel populations. Virginia Tech’s Freshwater Mollusk Conservation Center is perfecting lab methodologies for propagating endangered mussels for release into rivers and for use by other mussel hatcheries to enhance their propagation and restoration programs.
Mollusk center lab manager Dan Hua, a pearl culture expert from China and a part-time doctoral student in the college’s Department of Fish and Wildlife Conservation, has been helping lead the effort.
She works under the supervision of Jess Jones, a restoration biologist for the U.S. Fish and Wildlife Service based in the college and co-director of the center.
The mussel life cycle relies heavily on chance. The female’s eggs are fertilized when random males upstream release their sperm, and the female draws the sperm into her body while filtering the water for food. Once her eggs are fertilized, they develop into microscopic larvae called “glochidia” that must attach parasitically to a specific host fish’s gills or fins to continue their development.
Getting the glochidia to the correct host fish involves varying degrees of risk. Mussel species have evolved strategies for handling the transfer — from simply casting the larvae into the path of a likely fish to imitating a fish food and then clamping onto the fish’s gills while the larva stream aboard from the mother mussel.
A large mussel population improves the odds of these hit-or-miss placement strategies.
Hua, Jones, and their team of graduate students and technicians collect pregnant female mussels from the wild for propagation at the Freshwater Mollusk Conservation Center lab.
Hua removes the glochidia and places them in small tanks with the proper host fish, such as darters, logperch, and smallmouth bass, depending upon the mussel species.
Once the glochidia attach to the fish’s gills, the host fish are kept in cool tanks for two to three weeks while the glochidia transform into juvenile mussels and then drop off the host fish. The juvenile mussels are siphoned from the tank bottoms and raised in the lab until they are mature enough to return to the rivers.
In the early days of mussel propagation research, the juvenile mussels were released into streams within a month or two, soon after leaving the host fish.
“Sampling at the release sites showed the juveniles weren’t surviving,” Hua said, “but there was also a problem with getting them to live past a month or two in the lab; hence the need to release them at a young age.”
Hua came to Virginia Tech as a visiting scholar with a background in freshwater pearl culture technology, which has been developing in China since the late 1960s. Her first priority was to look at what the juvenile mussels were eating: commercially produced algae.
“I changed the juvenile culture methodology and developed new systems when I started my research as a lab manager under Dr. Neves in 2006,” Hua said. “The sterile environment, municipal water, and purchased algae protected the baby mussels from predators, but didn’t nourish them as they grew, so I started giving them pond water.”
Pond water is rich in nutrients, such as diverse algae, organic matter, and bacteria, especially when it is stocked with fish.
When Hua designed her pond system, she raised catfish to fertilize the pond water and added minnows to keep the zooplankton from eating algae the mussels needed.
She also added a biomedia in the fish holding systems and juvenile mussel culture systems in order to grow nitrifying bacteria to break down and neutralize the toxic un-ionized ammonia and nitrites that result from fish waste and decomposing matter.
Since mussels also need oxygen, Hua arranged for an aeration system to increase dissolved oxygen into her pond, a small outdoor pool beside the facility. The tiny mussels she suspended in cages in the pond grew much faster than they had in the lab.
“We could see them within two months,” Hua said. “Before we’d only been able to see mussels this age with a microscope, but these were significantly larger. Dr. Neves was very happy with my research. He called the mussel diets ‘Chinese ingredients’ and gave me more species to raise, including federally endangered species.”
They agreed Hua would raise juvenile mussels for a year or more before their release to increase their chances for survival.
“At first, I really didn’t want to let the mussels go. I kept some in a cage in the river to see how they did,” Hua said.
After investing so much time raising the mussels, Hua wanted to be certain that they would be released in a hospitable environment. An appropriate flow rate, depth, riverbed composition, and even the presence of old native mussels couldn’t allay her fears. To ensure that young mussels could survive at the potential release site, she tested the location by releasing young non-endangered mussels there and monitoring their progress before releasing any endangered mussels.
“Ideally, release locations should be pollution- and contamination-free, with the least amount of human disturbance possible,” Hua said. “Through trial and error, I found that by evaluating habitat and water quality assessments as well as using non-endangered juvenile mussels to test the suitability of potential release sites, we could identify those sites most likely to result in high survival rates.”
Sites along the Clinch and Powell rivers in Southwest Virginia and eastern Tennessee were chosen as release locations because they offered the greatest opportunity for successful restoration of mussel populations.
The emphasis is on protecting these rivers and others like them to sustain mussel and fish populations in the long term. Both rivers offer high biodiversity and the ecological conditions necessary for mussels and their host fishes to survive.
Hua’s work doesn’t end when the young mussels enter the river. One objective of her doctoral project is to monitor and model the survival and growth rate of released juvenile mussels to develop guidelines that can be used in mussel conservation programs.
Because recapture rates of released juvenile mussels using standard survey methods were so low (less than 10 percent), Hua developed a recapture method using passive integrated transponder (PIT) tags.
Now the larger mussels released often have PIT tags, which can be detected with a Geiger counter-like device. Using this method, her recapture rate soared to over 90 percent.
With grant money, the Freshwater Mollusk Conservation Center constructed a grow-out building where 20,000 to 30,000 juvenile mussels live year-round. Although mussels eat more and grow faster in warm water, Hua lets them experience a few months of winter temperatures in preparation for release.
Cold weather is hardly Hua’s only concern. “Pond water provides much more nutrition to juvenile mussels, but it can introduce predators, like the flatworm,” she said. “I quarantine all host fish in a special tank and use a 5-micron filter on the pond water. But flatworm eggs can enter, so we have to do weekly, labor-intensive sampling on all systems. We frequently move juvenile mussels to a clean new system. Fortunately, we only need to do this until the mussels reach a size of 3 millimeters or larger. At that point, flatworms can’t eat them.”
Hua adds that she often starts propagation for some species of mussels early in the spring when flatworms are very small and not able to harm the juvenile mussels.
Another problem is that host fish develop antibodies against the larval mussels that have a parasitic relationship with them. The lab must start with new fish for each new cycle of mussel propagation.
The system used for growing mussels to a large enough size for release is also critical. Following lab experiments with three types of growing systems — aquarium, bucket, and trough — Hua has settled on an open trough system that recirculates water from the pond. The system that she and her colleagues have adapted and perfected for freshwater mussels involves culturing the young juveniles in recirculating aquaculture stream systems, usually starting the tiny mussels in containers in the trough.
One surprise was the activity level of the tiny juvenile mussels. “They actually crawl from one container to the next,” Hua remarked. “Previously we weren’t aware of how many 1- to 2-millimeter juveniles were escaping, so now we have to put screens on the containers and use collection devices to keep the mussels from being swept away when we’re recycling the water.”
Hua and her colleagues have gradually developed propagation protocols for collecting pregnant (gravid) mussels, preventing and dealing with flatworm predation, holding host fish in captivity, maintaining water quality, and raising juvenile mussels to a size large enough for successful release.
When the mussels reach what Hua calls “tag-able” size — about 15 millimeters — they are ready to be released into the Powell, Clinch, or Nolichucky rivers in Virginia and Tennessee. More than 19,000 mussels raised at Virginia Tech — 90 percent of them endangered species — were released in 2012.
In October 2012, the Freshwater Mollusk Conservation Center, in partnership with Lincoln Memorial University, the Tennessee Wildlife Resources Agency, and the U.S. Fish and Wildlife Service, released more than 7,000 juvenile mussels into the Powell River, the largest single release of endangered mussels in the history of the river restoration project.
Each release site is assessed every one to two years. Mussels more than 2 years old are tagged so their growth and survival can be individually monitored.
Growing mussels can be a long-term commitment. Despite their strange, haphazard reproductive habits, mussels are surprisingly like humans in a way most other animals are not — under the right conditions, they can live 70 to 100 years.
So far, Hua says that 90 percent of them seem to be finding the right conditions to live well into their golden years.