Lake Vegetation Update

RRPJ-Lake Vegetation Update-17Aug23

Commissioner Faerie L. Sledge of the Grand Bayou Reservoir Commission, sent us this update on efforts to control vegetation on the lake:

Heavy rains in the Grand Bayou Reservoir watershed area the weekend of August 10, 11, and 12, flushed out large amounts of vegetation, including giant salvinia and water hyacinth, into the main channel of the lake. Some of it gradually floated down the lake and over the dam after the rains stopped, but some large areas still remain. Large pockets of vegetation still remain on both sides of the east end of the LA 784 bridge near Grand Bayou Resort. Smaller pockets remain in shallow coves in other areas.

Grand Bayou Reservoir Commission Chairman Mary Ann Wiggins spoke with biologist manager for District 10, Louisiana Wildlife and Fisheries (LDWF), Villis Dowden on Monday, August 14, 2017. He stated that residents can expect LDWF to increase the spray schedule this week in Grand Bayou. While state aquatic crews have already sprayed the lake several times this year, they will spray one or two times a week for the next month or so until the aquatic vegetation situation is back under control. Dowden also indicated that the warm winter of 2016-2017 probably made the salvinia problem worse.

Dowden explained that LDWF uses an integrated management approach to weed control: chemical, biological, and physical. All three methods are tools that can be used, although no one method works best on any one lake.

Chemical methods include spraying with a mixture of chemicals such as glyphosate, diquat, and surfactants. This is the method most often used on Grand Bayou. However, because salvinia and water hyacinths float on top of the water, they can be moving targets. Salvinia can appear or disappear in a particular area overnight during high winds, when the lake floods as it did in March 2016, or when heavy rains occur.

Biological controls may include salvinia weevils, although this method is still being studied. The weevil produces larva which burrow small holes within the salvinia. Trials have been done on nearby Lake Bistineau, but they have not been very successful. One problem with this control method is that many of the weevils cannot survive cold weather. Also, they can float over the dam and out of the lake after

being stocked. Grand Bayou Reservoir is a borderline point for cold survival of the weevils. Many of the waterbodies that have been stocked with weevils have dense tree cover, lack current, wind and wave action. Grand Bayou Reservoir typically has all three factors, but also has a few inaccessible areas that typically hold enough salvinia to consider a trial weevil stocking in the future.

Physical means of weed control include drawdowns. Drawdowns for weed control were done on Grand Bayou in 2006, 2007, 2008. (Salvinia was first found on the lake in 2006). Another drawdown occurred in 2015, but it was done at the request of the lake commission for bulkhead repairs. Drawdown is not being considered at this time for weed control on Grand Bayou according to Dowden. It is most helpful for submerged aquatic vegetation such as hydrilla rather than salvinia and could be considered by the Grand Bayou Reservoir Commission and LDWF if a 40% coverage area is surveyed. Since Grand Bayou Reservoir has a low amounts of woody cover for fisheries habitat, LDWF would like to keep anywhere from 20-35% total coverage of various submerged species composition including hydrilla.

Various experimental mechanical methods such as the salvinia “harvester” and the lake “mower” have not proven to be feasible due to the time and expense involved. Booms may be helpful in a few limited situations to keep the salvinia confined. The problem with using booms is that they might work until waves or current send the salvinia over the boom.

Dowden did explain that while Grand Bayou does have a salvinia problem, this lake will never have a problem to the extent that lakes such as Bistineau have due to the open water and occasional wave action. Bistineau has many more coves, sloughs, and trees than Grand Bayou Reservoir has.


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