Managing Giant Salvinia with Drawdowns Has Led to a Bass Boon at Bistineau

story by TREY ILES, LDWF Public Information

Giant salvinia can be a plague of biblical proportion, especially for biologists charged with managing the invasive species in Louisiana’s lakes and waterbodies. When the small fern finds its way into ideal habitat, it can smother a lake if left unchecked.

Take Lake Bistineau in northwest Louisiana for example. The plant was first found there in March of 2006. By 2009, the 17,200 acre lake was being swallowed by giant salvinia as the plant had spread over much of the lake and was estimated to completely cover nearly 7,500 acres.

Complicating matters, giant salvinia has the potential to double in biomass every 3-5 days during the prime growing season on Lake Bistineau and it is easily moved about by wind or currents.

As soon as the plant was found, biologists with the Louisiana Department of Wildlife and Fisheries began an assault on the free floating aquatic fern that is native to Brazil in hopes of completely ridding the lake of the plant. However, they soon realized it was too widespread for eradication.

So LDWF personnel decided to go with a three-prong attack of drawdowns, herbicide spray treatments and salvinia-eating weevils.

Though the unwelcomed plant remains, its dominance has been greatly reduced. The goal is to keep giant salvinia’s foothold to a minimum, allowing the public as much access as possible.

All three combatants in the war to defeat giant salvinia play an important part. However, drawdowns appear to hold the key in keeping the plant in check.

Prior to giant salvinia, Lake Bistineau was already under attack by Mother Nature. The lake had a declining fish population and decreasing water quality.

What was causing this decline? The answer is simply time said Jeff Sibley, Inland Fisheries Biologist Manager.

“Lake Bistineau was aging at an accelerated pace from years of stable water levels allowing nutrients and organic material to build up within sediments of the lake,’’ Sibley said. “This process, known as eutrophication, happens to all lakes to some degree. Fertile lakes in warm climates will age much faster.’’

The drawdown process exposes bottom sediments to oxygen and sunlight, increasing the decomposition of organic materials. This improves spawning habitat by solidifying the lake bottom, increasing available forage and releasing nutrients into the water when the lake refills. Improved spawning of all fish species provides greater food resources for the lake’s largemouth bass population.

Frequent drawdowns have played a major role in rejuvenating the once declining fishery. Lake Bistineau has now become one of the nation’s top bass fishing lakes, according to Bassmaster Magazine.

The magazine ranked Lake Bistineau No. 16 in the southeast in 2016 and No. 27 nationwide in 2015. With Toledo Bend, ranked first, and Caddo Lake, rated seventh, Lake Bistineau is in elite company as not only one of Louisiana’s best bass lakes but as one of the best in the country.

Crappie, redear and other sunfish have also increased thanks to drawdowns. Limits of crappie are common place from fall through the winter with plenty of one-pound plus slabs. A whopping 3.44 pound white crappie was caught in February 2015 and now ranks as the fifth biggest caught in the state. Local anglers, targeting bedding redear in April, are rewarded with full coolers and it is not uncommon to catch some fish pushing the one-pound mark.

“We began drawdowns on Bistineau about 10 years ago and we’ve seen the bass population steadily climb since then,’’ said Ricky Moses, LDWF Inland Fisheries Biologist Director. “We’re seeing positive benefits from these drawdowns.’’

The current management strategy is designed to reduce vegetation levels while maximizing the recreational opportunities that exist, including fishing, boating, duck hunting and other water sports, and also slow the eutrophication process.

Lake Bistineau is a large, diverse waterbody. Portions of the lake are heavily timbered with thick stands of cypress trees and some tupelos. There are also some large, open water areas that are favored by water sport enthusiasts. Much of the lake is shallow and lacks wind or wave action making it prime habitat for aquatic weeds. To further complicate matters, nearly a million acres of land drain into Lake Bistineau.

Herbicide spraying can be effective in some locations, but it is expensive. According to Moses, LDWF spent $3.6 million spraying on Lake Bistineau from 2009-2015.

“Our ability to spray is very limited on cypress/tupelo lakes,’’ Moses said. “We’re doing aerial herbicide applications when appropriate. We can cover a lot of ground and spray a big area quickly. But that has to be done in an open area. You just can’t sufficiently spray this stuff in the timber.

“The heavily timbered areas act as a refuge for salvinia to multiply. Then, at some point, we’ll see high water or wind moving it into areas of the lake where it causes problems for shoreline residents or at boat launches. ’’

Drawdowns kill salvinia by stranding it on the dry lake bottom and allowing the sun to dry it out. When the lake is drawndown, there is less prime habitat available for salvinia to grow in and spread into new areas. The plants remaining will be left in the more open parts of the lake which allows the herbicide treatments to be more effective even opening up the option for aerial spraying.

“We did an aerial application recently that covered about 600 acres,’’ Moses said. “They’re able to do that in two days. We can’t get anywhere near that with boats. When we can aerial spray, it’s very useful. It’s kind of expensive. But we can get a much quicker response.’’

Moses said the salvinia-eating weevils have produced good results in other parts of Louisiana. Unfortunately, the insects don’t reproduce or survive well during the winters of north Louisiana. LDWF is working with multiple researchers on developing a more cold-hearty weevil to help slow giant salvinia growth in Lake Bistineau and other northern Louisiana lakes.

“We feel like if we can get a population of weevils that could over-winter in sufficient numbers, we would see big benefits,’’ Moses said. “Our deal is we can’t just sit back and wait on that. We have to act right now.’’

And the best course of action is with drawdowns, which have proven the most useful.

Moses said drawdowns are more effective on Lake Bistineau when they begin during the early part of July. That allows the water bottom to be exposed to summer’s heat for nearly three months, eliminating the organic material in the lake bottom and greatly exposing giant salvinia.

Summer drawdowns are a necessity given Lake Bistineau’s large watershed. The ratio is 53 acres of runoff into one acre of the lake. All told, about 923,520 acres drain into the lake.

“That’s very large,’’ Moses said. When the fall and winter rains begin the lake will fill up, so we need to start fairly early in the summer with the drawdown.’’

Moses said LDWF knows drawdowns aren’t an ideal solution. But it is, by far, the best way to battle giant salvinia and also improve the habitat on Lake Bistineau.

“The good thing about the lake is that at 17,000 acres, even when we draw it down eight feet, which is our recommended depth, there is still more than 10,000 acres out there to use,’’ Moses said. “That’s still a pretty big lake. We understand it’s an inconvenience. But the benefits that are derived from drawdowns are so great we believe it’s worth it.’’


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