Weeds flourish in lakes; more grass carp proposed

So says Casey Moorer of the Biological Services unit of Santee Cooper, the state-owned utility that manages the lakes.

The nuisance weed can interfere with boating and fishing, especially along the shore, and it can clog intake valves at the electricity generating station.

Lakeside residents and businesses have noticed. About 500 of them asked Santee Cooper to spray herbicides in trouble spots last year, compared to 100 to 175 in a typical year.

A satellite multispectral analysis of a 2023 vegetation survey identified 8,491 acres of invasive species of all types on the lake system, Moorer said.

About 5,232 acres is primarily hydrilla, which often grows alongside Eurasian watermilfoil, which is the most recent invasive weed to infest the lakes.

Another 2,130 acres is primarily giant salvinia. “We’re still seeing steady increases,” Moorer said. But 100,000 giant salvinia weevil adults and larvae that were stocked in Lake Moultrie last summer might soon make a difference.

“Crested floating heart might actually be (the one invasive aquatic weed) we’re winning the battle against,” Moorer said. It’s down to 691 acres.

Native vegetation grows in an estimated 24,219 acres of the lake system. They play a key role in maintaining the health of the ecosystem, Moorer said.

Along with chemical control is biological control. Officials stock the lakes with non-native fish – triploid grass carp – that have voracious appetites for non-native aquatic weeds, especially hydrilla, but are unable to reproduce.

The introduction of the grass carp to the Santee Cooper lakes about 35 years ago was experimental and controversial. Some feared that the fish would harm the lakes, weren’t sterile, or wouldn’t devour lots of invasive weeds.

Over the years, the fish turned out to be beneficial, and the controversy has shifted to the right number of grass carp to stock in the lakes.

“We would stop stocking for several years at a time, and then we would have an explosion of hydrilla, and then we would stock 100,000 fish for two years in a row,” Moorer recalled.

The sudden influx “was detrimental to our native species that we’ve been working really hard to get established and protect,” she said.

Officials stocked 10,000 sterile grass carp in each of the past six years, “which was effectively decreasing our grass carp population at a fairly slow rate,” Moorer said.

“Last year we transitioned to a true maintenance stocking. That was always the goal when the program began, but we just never got there,” she said.

Last year, 11,025 grass carp were added to the lake system, bringing the total up to an estimated 34,451.

They were released at three sites on Lake Marion and two on Lake Moultrie “where we saw hydrilla start to pop up, so they wouldn’t have to travel very far to find their next meal,” she said.

But the hydrilla kept growing. “We talk all the time about not getting behind the 8 ball, but we’re there now,” Moorer said.

“In response to what we’re seeing with that significant increase in hydrilla acreage, our recommendation is to increase our grass carp population to just over 40,000 fish,” she said.

Taking into account the anticipated fish mortality rate, that means adding 16,580 grass carp to the lake system in 2024 to reach a ratio of one grass carp for each 4 surface acres of water. The lakes cover a total of 160,000 acres.

Moorer spoke during the South Carolina Aquatic Plant Management Council’s 143rd meeting, held at the Harbison State Forest on Tuesday, Jan. 23.

The council and the state Department of Natural Resources are responsible for the management of nuisance aquatic plants in the state’s public waters.

Each year they produce a plan that identifies aquatic weed problem areas, prescribes management strategies, and determines funding requirements.

In the program’s 43 years of existence, more than 208,000 acres of invasive aquatic vegetation in public waters have been controlled to improve habitat, public access, and use.

The council voted to include Moorer’s recommendation in the 2024 draft plan, along with the stocking of an additional 3,490 grass carp in 10 other lakes across the state.

The public is invited to read the draft plan online at https://www.dnr.sc.gov/water/envaff/aquatic/plan.html

Comments and suggestions on the draft plan can be submitted to invasiveweeds@dnr.sc.gov or SCDNR, Attention: Julie Holling, PO Box 167, Columbia, SC 29202. The deadline is Tuesday, Feb. 27.

Holling is the chair of the council and the manager of SCDNR’s aquatic nuisance species program.

Comments and suggestions regarding additional areas of problematic aquatic vegetation on the state’s public lakes and waterways are also welcomed.

The council plans to meet in early March to review the public comments and suggestions and to vote on final approval of the plan. Requests to receive notice of that meeting’s details, including WebEx and phone login information, should be directed to Holling.