How carp ‘won the race’ against invasive plants in Candlewood Lake

Like other experts, the Candlewood Lake Authority suspects grass carp are at least in part to blame for the lack of vegetation in Candlewood Lake.

Now, the organization is ramping up future monitoring and developing plans to help plants grow again in the lake.

“Plants are very important to a healthy lake ecosystem and we definitely do not want a situation where the lake is completely devoid of plants forever,” said Neil Stalter, Candlewood Lake Authority’s ecology and environmental education director.

His comments came during a Wednesday night webinar on Candlewood’s rapid plant loss. The thousands of sterile grass carp that were stocked in the lake in 2015 and 2017 to combat Eurasian watermilfoil ate not only the invasive weed, but other plants in the lake. It’s one of several factors that contributed to the loss of plant life, Stalter said.

When the carp program started, milfoil infested about 500 to 600 acres of the lake. Stalter said there were about 400 acres of milfoil in Candlewood in 2021, and that dropped to “effectively zero” in 2022.

In addition to investing in new technology for its research vessel so it can start conducting plant surveys earlier in the season and keep a close eye on what’s happening with aquatic vegetation, Stalter said the authority plans to build exclosures to keep out grass carp and give plants — ideally native ones — the opportunity to grow and reproduce in the lake.

Stalter said removing the grass carp is something the lake authority would like to avoid if possible, but the state Department of Energy and Environmental Protection is working to “remove some of the roadblocks for removal of the grass carp” in case it has to happen.

“If we’re seeing in the beginning of the season that we’re still in this state and the lake ecosystem does not seem to be changing, they will have everything arranged to potentially take that step,” he said.

Why the plants are gone

Stalter’s theory as to what may have led to the lake’s sudden loss of vegetation echoed those of experts who have said they believe the plant-eating fish put in the lake years ago are at least partly responsible.

“There are a lot of factors at play that impact the plant community and actually precipitated this major change,” Stalter said.

After the carp were introduced, the lake saw fluctuating levels of the invasive weed’s abundance for several years, followed by a sudden sharp decline.

Based on observed lake conditions, Stalter said several factors have likely contributed to its rapid loss of plant life — one being the presence and overfeeding of sterile grass carp.

“They’re in the lake, eating the plants and have grown to a larger size … and they are probably eating more than what was initially expected,” he said. “Probably more important than that, though, is they actually have been consistently reducing the total biomass amount of plant material over the past four years or so.”

Stalter said the total biomass of milfoil last spring was at a lowerthan-normal level, and the carp started feeding at a faster rate than the plants could grow.

“The carp effectively won the race against the plants and lowered the population to virtually zero,” he said, noting that the goal of stocking the lake with grass carp was to combat milfoil and lower plant biomass to a healthy level — not to eliminate all the plants entirely.

But there are a number of factors that pushed the level of lake biomass down to a lower level, which allowed the carp to “win the race.”

“The carp are growing and eating efficiently, they’re lowering the biomass consistently over the course of a few years, you potentially have an effective drawdown, you potentially have plant population changes and you have some climate factors that may be having an impact too,” he said.

The stocking of grass carp is one of two plant management strategies used in Candlewood — the other being annual winter drawdowns, which expose the shallow shoreline to cold air and make it harder for plants to grow the following season.

Stalter noted that the medium-depth drawdown of the lake in 2022 effectively exposed shallow plants to cold air and froze the root structures of plants that had existed the season before.

Natural “boom-andbust” cycles of plant communities may have also played a part in the loss of Candlewood’s vegetation, according to Stalter, who said the Candlewood Lake Authority is also looking into whether ice and snow coverage may have been a factor as well.

“We did have good ice cover on the lake for a little while last year, and that was then covered with snow,” he said. “What happens with that is the snow actually reflects most of the sunlight that would otherwise be entering the lake and potentially preserving some of the plant material over winter in the water.”

‘Push the ecosystem back’

Stalter said he looks at what’s going on through the lens of alternative stable states — an ecological theory that says an ecosystem tends to stay in one of two opposing states unless disturbed by an outside force or change.

When it comes to Candlewood, the two opposing states are: plant overabundance and very few or virtually no plants at all. Stalter said the lake’s ecosystem has historically sat in a stable state of overabundance, but it transitioned to a state of very little plant material this past year.

Grass carp stocking and drawdowns have been used to try to push Candlewood’s ecosystem to a point where it’s balanced with a healthy amount of plant material, Stalter said, but it’s “a tough balance to strike.”

“The ecosystem generally wants to fall back down — and historically it has — into a stable state of overabundance,” he said.

Plant management techniques usually aren’t enough to push the ecosystem to a balanced point, Stalter said — but this year, “the push was too strong.”

He said the grass carp, drawdown and other natural changes in the ecosystem appear to have pushed the ecosystem too far — causing it to reach a “tipping point” — and now that it’s transitioned to the opposing stable state, it may remain there until an outside force or change occurs.

“The trick now is to see what forces can still be used to push the ecosystem back … toward the other stable state while still trying to strike that balance between the two,” Stalter said.

He said “small factors” might be enough to bring balance to the lake’s ecosystem, but there’s no telling if or when that could happen.

“Forces that might begin to push it back in the other direction are the natural death of the grass carp, potential removal of the grass carp, less intense drawdowns, milder winters — all of the things that might give the plants an edge at the beginning of the season,” he said.