Exotic Aquatics...

Manna from Heaven or Weeds from Hell?

Please note the publication date of this article. Statistics and seasonal information were accurate at the time of publication. Check links provided for the most current information.

By Larry D. Hodge

Published in Texas Fish & Game, April 2004

Depending on your point of view, exotic aquatic plants such as hydrilla can be the best thing to hit Texas waters since plastic worms or the worst plague since Moses turned Egypt’s water to blood.

“Aquatic vegetation provides much-needed habitat for species such as largemouth bass,” says Todd Driscoll, TPWD district fisheries biologist from Jasper, who is also an active tournament angler. “In East Texas especially, probably 80 percent of anglers target vegetation. It’s good cover for adult bass and forage fish and also furnishes an edge that helps anglers locate fish. Without vegetation, you’re left with targeting deep structure, and it takes a lot more time to find fish.”

Aquatic plants in the right amounts can improve habitat for fish and provide food for waterfowl. However, too many plants can overwhelm a body of water. Thick mats of plants can impede water flow and boat traffic, clog water intakes for water and power plants, increase water loss from reservoirs, and lower dissolved oxygen levels by shading other plants and reducing photosynthesis. In extreme cases, overabundant plants can reduce the number of fish a lake can support.

While native plants are generally well-behaved, “Exotics tend to take over and occupy too much of a lake,” says Bob Lusk, a fisheries biologist from Whitesboro who advises private landowners on pond management. “I would take a native species over an exotic species any day of the week.” However, since almost all Texas water bodies are reservoirs, and not natural lakes, native plant species were not present in these reservoirs when they were first formed. Exotics do well in these new environments and often thrive to the point of becoming a nuisance.

The list of problem plants in Texas includes waterhyacinth (Eichhornia crassipes), hydrilla (Hydrilla verticillata), giant salvinia (Salvinia molesta), giant reed (Arundo donax), saltcedar (Tamarix), waterlettuce (Pistia stratiotes), alligatorweed (Alternanthera philoxeroides) and Eurasian watermilfoil (Myriophyllum spicatum). Giant reed (also called river cane) and saltcedar do not grow in water but do their damage by consuming huge amounts of it as they grow, lessening the amount available for recharge of aquifers and streams.

The vernacular definition of a weed as a plant in the wrong place fits every plant on the list above. All were brought to the United States by well-meaning people with good intentions. Water hyacinth, a native of South America, has strikingly beautiful lavender flowers and was introduced into Florida in the 1880s as an ornamental. Hydrilla was imported to the United States about 1960 for use in aquariums. Cultured in Florida canals, it escaped and is now found in nearly 700 bodies of water in 20 states, including at least 80 Texas lakes. Hydrilla spreads primarily by fragments carried from lake to lake on boats and trailers. Hydrilla grows in lower light conditions than native plants; it starts growing sooner and shades them out.

Water hyacinth and hydrilla have taken over stretches of the lower Rio Grande, forcing the Rio Grande Watermaster to release up to 30 percent more water from Falcon Reservoir to push water through to irrigators. The impact is felt upstream on Lake Amistad as well. Since a long-term drought has dramatically reduced the levels of both lakes, any loss of water is potentially damaging to the fishery, not to mention farmers and cities dependent on the water.

Is there a happy medium that can make fishers, boaters, swimmers, irrigators and operators of water treatment and power plants all happy?

“In my opinion you have to take hydrilla on a case-by-case basis,” says TPWD’s Driscoll. “I have 15 counties in Southeast Texas, and I only have four or five lakes I wish had no hydrilla. They are all small, shallow, municipal water supply lakes and are 50 to 80 percent covered with hydrilla. We deem 20 to 35 percent coverage as ideal, because if it exceeds that, it provides too much habitat and cover for bass and forage fish. Access for anglers can become restricted. Forage fish have so many places to hide that predators such as bass can’t catch them. Even though forage fish are abundant, bass will be skinny.”

Large lakes with little lakeside development and limited recreational use other than fishing can actually benefit from hydrilla, Driscoll says. “Water levels in Toledo Bend and Sam Rayburn fluctuate widely, and a lot of native plants can’t survive. Hydrilla abundance in those lakes accounts for most of the 10 to 20 percent vegetation coverage. Without hydrilla, coverage would be less than 5 percent.” Big bass in the grass give fishers a thrilla in hydrilla and make Toledo Bend and “Big Sam” prime fishing spots.

Bob Lusk says hydrilla might also benefit lakes that are built in canyons or steep, rocky terrain. Native plants aren’t adapted to such habitats, and deep water keeps hydrilla from blocking boat lanes. “Lake Amistad has such dramatic rises and falls that it can actually manage itself against the overwhelming growth rates of hydrilla, but for the most part I regard hydrilla as a nuisance,” he says.

While hydrilla has outright fans and earns reluctant acceptance from others, waterhyacinth has no friends. “Waterhyacinth is invasive in areas where it has no natural enemies and can occupy most of the surface,” says Lusk. “There’s no such thing as moderation with waterhyacinth. It’s either not there, or there’s too much of it. It sits on top and blocks out the sun, preventing other plants from growing. I have yet to see a situation where waterhyacinth is an asset.”

“From an ecological standpoint, we would rather have native vegetation that we would not have to treat with chemicals or non-native fish species such as sterile grass carp,” says Howard Elder, an aquatic vegetation biologist in TPWD’s Jasper office. TPWD is currently experimenting with plantings to establish native plants, and the results are encouraging, Elder says. That is a long-term, expensive solution, however.

Elder points out that hydrilla and other plants are prohibited, and it is illegal to possess, transport or transplant them, but he acknowledges that hydrilla is not going away. “It has reached such proportions that it is not cost effective to try to eradicate it in most places,” he says.

The best way to deal with exotics seems to be to control them where possible and avoid introducing them into new places. Private landowners with lakefront property needing vegetation control can request a permit from TPWD to treat water abutting their property. A TPWD permit is also needed to use sterile grass carp in private ponds. TPWD fisheries biologists will consult by telephone on aquatic vegetation control. For on-site assistance, landowners should contact the local office of Texas Cooperative Extension or a private pond manager.


Resources:

Managing aquatic vegetation and preventing the spread of problem exotics is a challenge for water body managers whether dealing with a small farm or backyard pond or a large public reservoir. These web links offer helpful information.

Notice:

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