By Elizabeth Pierson
Exotic fish may be the new warriors in the battle to control the growth of weeds that clog the Rio Grande.
Water officials plan to stock the Rio Grande with about 20,000 Asian grass carp in hopes that the fish will eat the hydrilla that chokes the river and has wasted hundreds of thousands of dollars in removal efforts. Because the carp are triploid, or sterilized at fertilization, scientists say there is almost no chance they will reproduce and damage the ecosystem.
The grass carp have been introduced in waterways across the United States to control hydrilla, and many biologists see it as a useful and responsible tool to control weeds that damage native ecosystems and local economies. But any exotic, or non-native species, put into an ecosystem should be viewed with a wary eye, some biologists say, especially at a time when there is a nationwide movement to restore native habitat.
“In general, it seems that often these types of introductions can cause more harm than good, and it can take many years for the unintended consequences to be realized,” said biologist Doug Jensen, coordinator of the Exotic Species Information Center at the University of Minnesota. Jensen works with a state and federal program to control and eradicate the 161 exotic species in the Great Lakes region.
Jensen, who has visited the Valley to see his parents in Donna, said he does not think that the introduction of sterile carp is necessarily a bad idea, as long as local experts have considered the consequences, particularly because exotic carp can be rough on an ecosystem.
“Carp can cause some devastation on water quality in general by stirring up sediment and uprooting vegetation,” Jensen said. “And I happen to know from personal experience that water quality is already an issue in the Rio Grande.”
But some scientists, including those who recommended the introduction, have fewer doubts about putting carp into the river. Scientists say they carefully determine the number of fish introduced. Most importantly, proponents say the process by which the triploid fish become sterile is virtually 100 percent effective, making it nearly impossible for the fish to overtake other species in the river.
The cost of leaving the hydrilla to grow in the river is greater than the potential cost of introducing the carp, project proponents say. Hydrilla also is a non-native organism, and allowing it to spread could worsen agricultural and municipal water shortages and further inhibit the flow of water down the river.
“This is not a fly-by-night plan,” said Ken Jones, administrator for the Rio Grande Regional Water Planning Group, which formed a binational committee to figure out ways to fight the hydrilla. The grass-carp introduction was one of the committee’s suggestions.
“This is something that has been studied carefully,” said Jones, noting hat this will mark the first time that grass carp have been introduced in an international waterway.
Research on how the fish respond to saltwater varies, and researchers wanted to be sure the fish would not spread to the Laguna Madre or the Gulf of Mexico, according to a report from Southwest Texas State University researchers. The researchers put 25 grass carp, fitted with transmitters, into the river in October 2001 and monitored their movement until May 2002.
The grass carp, which were placed in the river near Progreso and Granjeno, below Anzalduas Dam, stayed in the river and traveled an average of 15.3 kilometers during the study. They moved upstream, but generally stayed away from saltwater and the large dams.
Two grass carp in the study made it 26 kilometers upstream and went through Retamal Diversion Dam.
The study concluded that the grass carp can move through large, physical barriers, but that they would do well in the Rio Grande as part of a multi-faceted weed-control plan, said Tom Arsuffi, aquatic ecologist at Southwest Texas State University and an author of the study.
“The hydrilla is a tremendous problem and there’s a whole bunch of different ways you can approach containing it,” Arsuffi said. “I think using an integrated management approach, including biological controls like the grass carp, is a good way to go.”
The study convinced White and other members of the Aquatic Weed Task Force that the carp introduction would be environmentally sound. And the water managers can’t afford to leave the hydrilla. Irrigation districts have spent hundreds of thousands of dollars collectively on removal of the weeds, which have clogged their pumps. Some pull the weeds out mechanically; others use chemicals or grass carp. But no one method has been effective and cost-efficient for the districts.
During peak irrigation season, Rio Grande Watermaster Carlos Rubinstein said he has to release 15 percent to 20 percent more water than necessary from the Falcon and Amistad dam system to break through the weeds that clog the river.
Rubinstein is anxious for the day the carp begin eating the hydrilla so that once wasted water can go to the irrigation districts rather than running to the Gulf unused.
“All of us benefit from improving the efficiency of our ability to move water on the river,” he said.
The federal government spends more than $138 billion a year trying to control non-native species, but preventing their spread is largely overlooked, Jensen said.
“The amount of money dedicated to outreach is minimal compared to what we pay for other environmental issues,” he said.
The Rio Grande carp introduction will be paid for by a $150,000 grant to the Lower Rio Grande Valley Development Council from the National Fish and Wildlife Foundation. Local water districts have agreed to give a combined $300,000 in in-kind matching grants for the parts.
At between $7 and $8 per fish, the grant money should buy at least 20,000 fish for the river. They will go into the river at the same time, or may be introduced in successive weeks if there is time before the April 30 deadline set by the National Fish and Wildlife Foundation to receive the grant money, Jones said.
But before the fish can be put into the river, Texas Parks & Wildlife Department must sponsor a meeting to hear public comments, and the U.S. Bureau of Reclamation must file an environmental study.
The public comment session is scheduled for March 3 from 5-7 p.m. at the Weslaco City Hall.
Although native to the rivers of eastern China and the former Soviet Union, the grass carp are hardly strangers to the United States. The U.S. Bureau of Sport Fisheries and Wildlife introduced the carp in 1963 to help fight aquatic weeds in the Southeast. They have been eating hydrilla across the country since then.
Scientists have mastered the triploid sterilization process so that it is virtually 100 percent effective, said James Shelton, associate professor of fisheries with the University of Georgia who has studied public and private grass-carp introductions since 1986.
The triploid carp get their name because they have three chromosomes where their reproducing neighbors would have just two. Just after the eggs are fertilized in a laboratory, they are shocked and the fish are left sterile. Each fish undergoes a blood test to confirm its sterility, Shelton said.
“We have not documented any cases of a triploid fish reproducing here,” Shelton said.
The fish usually live from eight to 12 years and can grow in size to 70 or 80 pounds.
Unlike other carp, the grass carp are not considered bottom-feeders. Its mouth is in the front of its face, not on the underside, making it easier for grass carp to eat vegetation in front of it that grows from the bottom, such as hydrilla, Shelton said.
The carp will not disturb other nesting or spawning species, Shelton said.
They like hydrilla above all, according to Shelton’s research.
“They’re going to eat what they like best until it runs out, then they’ll move to the next plant on the list,” Shelton said. It is possible the carp will never eat all of the hydrilla in the Rio Grande, but rather help control it, said Jo Jo White, head of the Region M Aquatic Weeds Task Force.
“The extent of the hydrilla in the river now is such that even with the fish being introduced they would not completely devour it all; they would just keep it back,” White said. “This hydrilla is here to stay.”
Some studies have shown that grass carp like other plants found in the Rio Grande, such as duckweed and pennywort. But because the grass carp live only about a decade and the plants they eat are plentiful in the Rio Grande, there is no danger of those plants losing ground, said Alfred Richardson, a plant taxonomist at the University of Texas at Brownsville.
“Without reintroduction into the water systems, I don‚t see how the fish could overtake any ecosystem in that amount of time,” Richardson said.
At least some of the fish will likely be caught by humans looking for food in the river, which is what may have happened to the five fish that were unaccounted for at the end of the pilot project, Arsuffi said. The fish are large and edible, and are considered a delicacy in many parts of the world.