– The Louisiana Department of Wildlife and Fisheries is set to begin a drawdown of Henderson Lake in an effort to combat invasive plants that clog waterways and hinder the natural life cycle of the lake, a biologist with LDWF told the St. Martin Parish Council’s Public Works Committee this past Tuesday.
Lowering the level of the lake is one of three methods the LDWF uses to control the water hyacinth, giant salvinia and hydrilla that can reproduce rapidly and interfere with the biological health of the lake, Brac Salyers told the committee.
LDWF also wants to control the plants to help out crawfishermen, fishermen, oil and gas companies, swamp tours, boaters and others who use the lake for commercial or recreational purposes.
Salyers works in the wildlife and fisheries’ Lafayette office and has been the fisheries manager for Henderson Lake for the past 13 years.
Hydrilla is a submerged plant that can grow in water from a few inches deep to 25 feet deep, Salyers said. Parts of the plant cut off by boat propellers can float off and establish new plant colonies, he added, and covered 50 percent of the lake as recently as 2014.
Water hyacinth was introduced into Louisiana at the Cotton Expo (World’s Fair) in 1880 and has since moved into waterways throughout the southeastern part of the U.S.
Hyacinth mats can block sunlight in the water, hurting the ecosystem by preventing plankton from growing. Plankton provides oxygen in the water and is a food source for fish and other wildlife.
Water hyacinth can double its biomass in about 12 days, Salyers said.
“All of the Southeastern states spend a tremendous amount of money trying to battle that plant,” Salyers said.
LDWF treats 3,400 acres of water hyacinth in the 5,000-acre lake each year.
Giant salvinia was first observed in the lake in 2012. It usually fills the backwoods along the lake during mild winters, then begins filling the lake and interfering with boat ramps in the spring. It can double its biomass in 7-10 days.
“Coverage expands each year and then goes back a little bit,” Salyers said. “We always see an explosion in the spring.”
Drawdown
The LDWF uses an integrated control effort that features biological, mechanical and chemical ways to combat the three plant species, Salyers said.
The mechanical method involves using the Henderson Lake control structure to slowly lower the depth about 2 to 4 inches per day until it gets down to 6 feet. The hot summer months keep the oxygen levels in the water low, so reducing the depth of the lake more quickly could result in fish kills, Salyers said.
“That’s the last thing we want to do is create a fish kill by drawing the water down too fast,” Salyers said.
The drawdown can only begin when the water level is at 9 feet or lower, because of the level of the control structure.
The goal is to strand the hyacinth and salvinia plants in trees, shrubbery and along the banks, killing them. Those plants that root down into the sediment also will die the water levels rise again because the sunlight can’t reach them and they can’t photosynthesize nutrients, Salyers said. Exposed hydrilla roots also are vulnerable in the wind and sunlight over several years time.
The compaction of the soil at the bottom of the lake from two to three feet to six inches deep also helps the lake ecosystem, he said, by helping convert inorganic material in the mud to organic material that is used as nutrients for fish, plankton and other animals in the spring.
But the efforts that began annually in 1996 are affected by many factors such as flooding, heavy rains and the need to make repairs to the control structure, so the last fully successful efforts to control the plants via drawdowns were in 2014 and 2016, with 2014 making a big dent in the hydrilla problem that the species hasn’t been able to overcome since. LDWF has achieved partial success the past four years.
LDWF has made no effort to have drawdowns some years because of a lack of public support. The department won’t attempt a drawdown if it has a direct economic impact on someone who is making a living off the lake, Salyers said, but LDWF dredged lanes in the lake in 2012 that allow swamp tour businesses to use the lake during drawdowns, so efforts have been more consistent since then, other conditions permitting.
“We’ve been trying this since 1996,” Salyers said. “A lot of years we didn’t have the support. Now that we have the support, there’s still a lot of environmental factors in play and it doesn’t mean you’re going to have a successful drawdown just because you try. We just have to try the best we can and hope for the right environmental conditions.”
This year the department plans to have the drawdown from Aug. 1 to Nov. 1.
The drawdown is done in a slow manner to avoid fish kills and other adverse affects, Salyers said, but another benefit to this year’s drawdown would be aiding in efforts to repair the control structure as divers would have shallower depths to work in.
Biological control includes introducing grass carp into the lake, which eat hydrilla plants. The department has stocked around 53,000 grass carp over the last five years around Kern’s Landing, McGee’s Landing and Whiskey Bay, primarily, in the flats north and south of I-10 where the hydrilla historically has been the worst, Salyers said.
Giant salvinia weevils are used to control those plants, with around 300,000 released over a 13 year span.
“We probably will have to stock more grass carp, and we definitely will stock more giant salvinia weevils,” Salyers said. “It’s just management (of the plants). That’s the only way to keep ahead of these plants is to try to keep using this integrated approach.”
Because of budget cutbacks, LDWF no longer has dedicated spray crews for chemical treatments, so must hire contractors to spray EPA-approved aquatic herbicides on the plants, Salyers said.
The department has spent about $150,000 spraying this year already to control giant salvinia and water hyacinth.
Parish Council Chairman Chris Tauzin said that more fish than ever are being caught in the lake now, but a lot are small. He asked if size limits and stricter possession limits could be established for the lake to prevent overfishing and hurting the fish populations.
Keeping smaller fish hurts the population of good fish the next year, he said.
Salyers said LDWF is trying to get a feel from the public on its desires in the matter.
“If they’re keeping smaller fish that’s kind of a waste on their end, because how much meat are you going to get off of a 4-inch saca- lait, but there is a possession limit, and if they catch more than that limit, they’re breaking the law,” Salyers said.
Enforcement agents are writing tickets, and biologists conducting surveys will report boats they see that are carrying more than the limit, he added.
Tauzin also asked if more spraying could be done on the northern end of the lake where the water hyacinth seems to be coming from this year.
Giant salvinia is a big problem in the lake, he added, by hydrilla is not much of an issue right now.
“There’s not a lot of hydrilla,” Salyers said. “Each year when we do the drawdown, I see it along the Dixie, it’s always down by Basin Landing in a few places, but it’s nothing like it was.”
“No, it’s not even close,” Tauzin said.
“To be honest with you, I’d like to keep it that way,” Salyers said. “I know some of the bass fishermen do like patches of it, but it’s so incredibly hard to just keep a little bit of it. A little bit of it can turn into a lot very, very quickly.”
