In recent years, a tropical plant called giant salvinia has overtaken and overwhelmed some areas of Lake Bistineau, relegating a north Louisiana recreational paradise into an unnavigable swamp in some areas.
Through the use of various techniques, the Louisiana Department of Wildlife and Fisheries is working to limit the plant’s growth.
Unfortunately for Lake Bistineau, science says there is a perfect storm of factors currently working in the plant’s favor.
In most lakes where giant salvinia grows, the vegetation doesn’t get out of hand because wind pushes the water-bound plant onto the bank where it dries out and dies. However, the heavily forested areas surrounding Lake Bistineau stop most of the wind from reaching the water.
Thanks to the canopy they create, the trees also protect the giant salvinia from freezes, according to LDWF Aquatic Vegetation Program Manager Alex Perret.
Add the fact that Lake Bistineau is located in the portion of the state where the temperature gets cold enough in the winter to kill the insect that eats the giant salvinia, but not cold enough to kill the vegetation itself. More on our biological efforts to curb that trend later.
In 2009, giant salvinia spread to about 8,500 acres of the 17,000-acre lake, rendering boating practically impossible in an area known for its enjoyable bass and crappie fishing, duck hunting and water skiing. There’s also the cosmetic factor that people who bought houses along the water paid for water-front, not vegetation-front property.
For those reasons, LDWF has spent much effort to make strides in the better part of the past decade to lessen the amount of giant salvinia on Lake Bistineau through mechanical and chemical efforts.
Mechanical effort is executed through drawdowns, a method in which we drop the water level of the lake up to seven feet. This exposes the bottom of the lake in certain areas, drying out and killing much of the giant salvinia – a plant which typically grows in shallow waters. However, the trees once again come to the rescue of some of the vegetation, forcing it into areas where the lake is deeper than seven feet.
Perret said LDWF initiates the drawdown every year when vegetation encompasses approximately 1,500 acres of the lake. This usually happens around Labor Day, but LDWF attempts to keep water levels high so that all boat ramps and fishing areas remain open as long as possible for maximum public use. Unfortunately, that’s not always possible due to the rate of the vegetation’s growth.
“When we do a drawdown,” Perret explained, “hot weather is preferable. It dries the plant out, and it also provides benefits for the fishery. It dries out the lake bottom, which usually contains lots of organic matter, and lets it break down and decompose. It also compacts the soil, which improves spawning for your nesting fish like bass and sunfish. We have seen improvement in the last few years in the fishery sampling, likely due to regular drawdowns.”
Perret added that the drawdowns mimic normal yearly fluctuations in water level that occurred before humans impounded it with a natural structure.
But because drawdowns don’t substantially limit the plant’s growth the following year, lowering the water level is only a temporary solution.
When we fight giant salvinia with chemicals, we spray herbicide directly on the plants. All herbicides used are approved by the EPA through rigorous testing to make sure that there are no residual effects to the water or to the fish or wildlife that live in or near the water.
LDWF herbicide crews spend a great deal of time on Lake Bistineau, taking care of chronic problem areas or responding to camp or home owner complaints.
When large areas become covered in vegetation, typically in May, LDWF hires contractors who spray on and off in weekly stints as needed, usually until about November. After the plants are sprayed, field biologists follow up to make sure they’re dying and if any areas were missed and need to be treated.
But because the plant grows so quickly, spraying herbicides is also just a temporary solution. Plus there’s always the giant salvinia’s best friend – the trees – which grow close together in certain areas and restrict spraying access.
