Canaries in the Water
Mussels and fish tell us what is going on in rivers and streams.
Story and Photos By Larry D. Hodge
When you want to gauge the health of a stream,look at what lives there.
Or died there.
Aquatic insects, crustaceans, worms and mollusks that live in streams are widely used to determine a stream’s biological condition. These organisms can be found in all rivers and streams, even in the smallest streams that cannot support fish. Because they are relatively stationary and cannot escape pollution, these critters can be compared to canaries in a coal mine in detecting unhealthy conditions. A healthy stream supports a diverse assemblage of organisms ranging from microscopic plankton to bottom-dwelling mollusks to free-swimming fish. An unhealthy stream will be lacking many of the organisms that once lived there.
An Environmental Protection Agency national assessment of streams conducted in 2008-09 found that more than half the nation’s streams were not in good condition. In the Plains and Lowlands region that includes Texas, only 16 percent of the streams were rated good.
Some of the least-visible animals inhabiting Texas rivers and streams are the freshwater mussels. Texas is home to 50 of the 300 mussel species found nationally, and many of them are in trouble. In 2009, Texas placed 15 mussel species on the state list of threatened species.
Mussels’ obscurity belies their importance. When it comes to the health of a stream, if mussels ain’t happy, nobody’s happy.
The Texas fawnsfoot is one of 15 species of freshwater mussels listed by Texas as threatened. Though small, mussels play a huge role in water quality and stream health.
Mussels typically live out their lives partially buried in the bottom of a stream. Although they have a foot they can use to move about, they mostly stay put, letting the water bring food to them. Mussels are bivalves, meaning they have a shell composed of two hinged parts. When they feed, they open the shell slightly to pull water in through a siphon. As they extract oxygen from the water through their gills, they also pump water through their bodies and filter out food.
“Mussels are filter feeders that eat bacteria and algae and also remove pollutants from the water,” says Neil Ford, professor of biology at the University of Texas at Tyler. “A large bed of mussels can remove a significant amount of the sediments that come off the land, so they are very valuable just in terms of cleaning the water.”
Mussels can help shape the character of a stream as well.
“Some people believe that where you have large concentrations of mussels, they may help to stabilize the stream bottom because of the way they anchor in,” says Andrew Blair, a biologist for the Texas Department of Transportation’s Environmental Affairs Division, adding that mussels are also important for other macrobenthic invertebrates such as insects and worms. “They may provide microhabitat that would not be in the stream without them. All those macroinvertebrates are fish food and contribute to the diversity of organisms in the stream and ultimately to what fish species you see.”
Fittingly, the lives of mussels and fish are inextricably linked: Freshwater mussels reproduce by using fish as temporary hosts for their young. Female mussels use a variety of strategies to “infect” host fish with the larval stage of their young, called glochidia. Some mussels attract fish by opening their shells and waving a bit of tissue like a lure. When a fish comes near, the mussel snaps its shell closed, grabbing and holding the fish while it spews glochidia into the fish’s gills, where the glochidia attach using hook-like structures. Other mussels produce packets of glochidia that look like fish eggs. When fish eat them, they break open, and the eggs attach to the fish’s gills. Some mussels just eject their glochidia into the water when a fish is near, giving them the chance to attach to the fish’s gills or fins.
And in most cases, not just any old fish will do. It has to be the right kind of fish, or its immune system will kick in and reject the glochidia, which will die. The outcome is different if the right fish comes along at the right time.
“The larvae spend a short amount of time on the fish and drop off, become a juvenile and then grow into an adult mussel,” says Charles Randklev, an assistant research scientist for the Texas A&M Institute of Renewable Natural Resources. “This is unique, and it poses some serious complications to conservation. Not only do you need to worry about the freshwater mussel, but you also need to be concerned with their hosts. In some cases the host fish for a particular species is very specific. In Texas, those that are species-specific tend to be the ones that are most imperiled.”
Humans and nature have teamed up to do a number on mussels.
“In Central Texas we have the issues of drought and the dewatering of some rivers,” Ford says. “If a river goes completely dry, the organisms that live in it can’t survive. In East Texas, dams are more of an issue. Dams change the flow systems of rivers such that you get more siltation, which covers up the beds of mussels. Building reservoirs also impacts the ability of fish to move and disperse the juvenile clams. The other thing is that dams change the nature of a river for fish. A dam may release colder water during the summers, and instead of having a warm river, you end up with a cold one. When you change the temperature and the oxygen levels below dams, you may change the fish, which can then change the mussels.”
Most dams in Texas were built following the drought of the 1950s, and their impact on river species may not yet be fully known.
“Mussels live an incredibly long time — some we think 80 years,” Ford says. “A lot of them reproduce in the summer, and if a dam changes the water temperature, you may affect reproduction. We may find lots of adults of a particular species but not any juveniles. If the mussels are not recruiting any young, we may have an extinct species and not even know it.”
Think dead mussels “not” walking.
Ford has been studying rivers in East Texas for several years, and the outlook for mussels there is mixed.
“The Trinity appears to have a better mussel fauna than you might expect,” he says. “Wastewater treatment plants in the Dallas-Fort Worth area may have saved the mussels. Many places in rivers dried up during the drought of 2011, but the Trinity had water flowing all the time because of the water coming from the treatment plants. We found a number of beds right below treatment plants, and I think that says something important about our treatment facilities. The water that is released is pretty good, at least for mussels.”
Mussels in the Neches River are doing well for different reasons.
“Probably the No. 1 problem for East Texas rivers is erosion,” Ford says. “When a river is dammed, you change the nature of the flow, and water tends to undercut the banks, which fall in. If you accompany that with farming right up to the edge of the river, you see a lot of bank erosion, which results in siltation. Sand comes down, buries the mussel beds and kills them all. The Neches is pretty well protected from that; its riparian zone is wide, since some of the land around it is in tree farms. The other thing is that the banks of the Neches are low, so during floods it overbanks, and the water goes out into the floodplain. That reduces shear stress in the river, which if high can rip mussels out of their beds.”
Ford says having instream flow requirements below dams would help ensure that mussels get constant water flow, but even that might not be enough.
“One of the problems with dams is that the nutrients in the water fall out to the bottom,” he explains. “If it’s an overflow-type dam, the water coming off is nutrient-poor, and there’s nothing for mussels to eat. It’s very important to understand the ecology of how the dams work, how the rivers work, the fish and the mussels, and how they all intertwine in terms of survival of animals downstream. If we can get water for human needs without changing the nature of the river, then we can save what’s there at the same time we solve our water problems. If we know enough. And that’s the deal. We’ve got to understand the ecosystems better than we do right now.”
What would rivers look like without mussels?
An Environmental Protection Agency national assessment of streams conducted in 2008-09 found that more than half the nation's streams were not in good condition.
“I’ve gone to places where there are no mussels, and the river usually doesn’t look very good,” Ford says. “There may be a lot of algae on top, or it may be muddy. Those systems have been damaged, and the mussels aren’t there. My feeling is if they are gone, the reason they’re gone is what you are going to see.”
The work of Ford, Randklev and others shows that many Texas streams have already lost some or all of their mussels. Can they be brought back? Maybe, maybe not. The study of mussels is at such a rudimentary stage that the host fish for most species in Texas is not known.
Mussels from the San Saba River being measured and tagged so their growth and survival can be monitored over a two-year period.
“A fair number of the host fish are predatory fish, like bass or sunfish, so what you have to do is find a pregnant female mussel, bring it in, maintain it in captivity — which requires flowing water — then add the fish and try different fish to see if the glochidia take,” Ford says. “It’s been done enough in other states so that we can probably guess what some of the hosts are going to be, because we have a related mussel somewhere else. A single female mussel can produce a huge number of offspring, so you can get large numbers of mussels and grow them up fairly quickly to a size that can be put back into the environment.”
However, Ford cautions, it won’t be easy. For one thing, you need hatcheries that can raise mussels, and the five Texas state fish hatcheries are already unable to meet all the demand for fish stocking. And then there’s the question of where to put mussels once you’ve raised them.
“The trick is you have to have good habitat available, and the only habitat available may be habitat they are already in,” Ford says. “If we can create habitat or improve habitat that has been degraded, I think there are some real opportunities for mussels to come in naturally or for us putting some species back ourselves. Luckily, only a few species are at a state where we need to do this, but with the continued demands on our rivers and the possibility of droughts, we need to be planning ahead.”
You may never have seen a mussel, but how you manage your lawn in Lewisville or your farm or ranch in Tom Green County impacts them. Everything that happens in a watershed affects its rivers and streams and all the things that live there. The mussels and fish that live in those streams are doing their best to make things better.
Are we?
Monitoring MusselsTexas has about 191,000 miles of rivers and streams and some 1,000 reservoirs, so learning what mussels are out there is a daunting task. The Texas Mussel Watch is a citizen science program that uses volunteers to find, identify and monitor mussel populations. “The program started in 1997, because we knew mussels were becoming rare, and we didn’t have enough biologists looking for them,” says Marsha May, program coordinator. May conducts three to five workshops annually to train volunteers, and she sees the program as more than just a way to collect important data. “I think it’s important to educate people that we have freshwater mussels in our systems,” she says. “That’s the first step in protecting them, because in order to protect them, people have to appreciate them. Sometimes I have trouble getting people to come to a workshop, but when they leave they are saying, ‘Wow, mussels are amazing.’” Who should care about mussels? Everybody, May says. “They are the vacuum cleaners of the river systems and the foundation of the aquatic ecosystem. Research is now being done to prove their importance in cleaning drinking water. Past research has shown that habitats with lots of mussels have more abundant macroinvertebrates that are food for small fish. And who eats small fish? Bass.” If you would like to learn more about mussels or become a Texas Mussel Watch volunteer, visit www.tpwd.state.tx.us/mussels. |
