Weed Woes
Giant salvinia threatens east Texas reservoirs.
By Larry D. Hodge
One of the world’s most noxious aquatic weeds has invaded one of Texas’ foremost largemouth bass fisheries and is poised to attack another.
Even the name of the plant is sinister: Salvinia molesta, commonly known as giant salvinia. Under optimum conditions this floating fern, a native of South America, can double in size weekly. Its growth on Toledo Bend Reservoir gives a clue to its aggressiveness. In 2003 it covered 124 acres on the lake; in 2004 it spread over 3,070 acres despite ongoing herbicide treatments by both Texas and Louisiana. Sam Rayburn Reservoir may be the next target.
Giant salvinia forms thick floating mats that block sunlight and prevent the production of microscopic organisms vital to healthy fish populations. Under good growing conditions the plant can produce nearly 100 tons of biomass per acre, and once the floating mass dies and sinks, the decomposing material can use up all the oxygen in the water. TPWD aquatic vegetation biologist Howard Elder fears the plant will take over shallow coves where largemouth bass spawn and seriously impact the fishery.
First discovered in Texas in 1998, giant salvinia was probably sold for use in water gardens by nurseries that had no idea its importation or possession is prohibited by both the United States Department of Agriculture and TPWD. “One good flood empties out somebody’s backyard and infests the whole watershed,” says Elder. “Giant salvinia is easily transported overland to new locations by boat trailers, propellers and the intakes of personal watercraft. Considering the proximity and popularity of Toledo Bend and Sam Rayburn, its introduction to Sam Rayburn must be considered inevitable.”
TPWD and the Sabine River Authority have been battling the invasion with herbicides, but the rains of 2004 kept Toledo Bend Reservoir full and allowed the plant to spread into shallow, stump-filled areas where spraying boats can’t go. “In 2004 it overwhelmed us. We were able to treat only 228 acres,” Elder says. “Our goal is to keep it contained in Toledo Bend and keep it from reaching Sam Rayburn. We are concerned that some anglers and boaters use both Toledo Bend and Sam Rayburn and trailer boats from one lake to the other. We are placing signs at all major boat ramps on Toledo Bend to inform the public of the dangers of giant salvinia and requesting boaters to clean their boats and trailers of all aquatic vegetation before leaving the ramp area.”
Elder notes that if you do not remove giant salvinia from your boat or trailer before you leave the lake, you can be charged with possessing and transporting harmful exotic plants. These charges carry penalties of fines and/or jail time.
TPWD has begun large-scale introductions of a biocontrol agent, the salvinia weevil Cyrtobagous salviniae, which feeds on the plants and may have long-term potential. “We are optimistic establishment will occur in spring of 2005, and we will continue stocking them. I hope they can do the job, but even then we may be looking at one to two years before they reach the point where they will do any real good,” Elder says. “In the meantime, we can expect another increase in the expansion of giant salvinia on Toledo Bend.”
