Thousands of invasive carp removed from Silver Lake

Silver Lake has a carp problem.

Carp, an invasive species introduced to the United States in the mid-1800s, stir up the bottom of the lakes they live and spawn in, reducing water quality in the process.

As carp populate a lake, they eat by rooting around in the mud at the bottom, uprooting plants and releasing trapped nutrients. This influx of nutrients makes it easier for algae to grow in the lake, shifting the lake from relatively clear water populated with native plants to relatively muddied water populated with “pond scum.”

None of this is new. In 2015, a group of fishermen removed about 6,000 pounds of the fish from the lake. But, largely due to the continued carp presence, the lake’s water quality is still below what some residents hope for.

The Rice Creek Watershed District (RCWD) has been instrumental in monitoring water quality in the metro area. Matthew Kocian, aquatic ecologist and project manager at the organization, spoke about the lake with the Northeaster.

“Looking back at the last ten years,” Kocian said, Silver Lake’s water quality “is somewhere in the middle. It’s been jumping back and forth… We’re right at the tipping point of it being better, I hope.

“Silver Lake has had too many carp for a number of years… I think there’s probably still some more work to be done out there in terms of carp management, but I think we’ve taken some big steps this year.”

One of those big steps includes a recent removal effort by the RCWD and the Silver Lake Residents Association (SLRA). Roughly a half-dozen individuals, equipped with netting to capture both large and small carp, went into Silver Lake and netted about 6,000 fish. Combined with previous efforts from this year, Kocian says that approximately 8,700 carp have been removed from the lake.

That said, Kocian stressed, it’s an uphill battle. “One female carp can lay one million eggs,” he said. “So if you’re successful — if you spend a whole bunch of time and energy netting the carp population — it wouldn’t take very long for the carp population to rebound.”

“Winterkilling” compounds the problem. If a lake freezes over in the winter, this limits the dissolved oxygen available to fish and plants under the surface. In Silver Lake and other relatively shallow lakes, Kocian says, the native plants are the first to die. The carp, by contrast, are “tougher… They’ll survive down to a lower dissolved oxygen threshold. They can last longer.”

As a result, the fish that would normally eat the carp’s eggs die in the winter. A harsher winter, in other words, means better odds for the carp in the following year. A few years ago, an aerator in the lake malfunctioned, making a winterkill more likely in the lake. This year’s removals, Kocian said, likely come from eggs laid that year.

“It’s going to be really important that we maintain (Silver Lake’s) aerator in the future,” Kocian said. If they’re able to keep the lake habitable to native species, “when the carp spawn in the lake — and they will — the native species will eat the carp eggs and naturally control that population.”

Kocian added that the RCWD is considering installing a grate or other simple barrier to prevent the carp from migrating between lakes. “When carp can migrate to shallow lakes at winterkill, they can spawn there,” he said. “Their eggs are safe from predators, and then the adults can move back into the deep lake later.” A grate could help keep the carp contained.

April Chu, member of the SLRA, said the carp removal has been “far beyond what anybody was anticipating.” Early estimates said the lake likely had hundreds, not thousands, of the fish.

The SLRA, Chu said, is working on introducing native plants and animals back into the lake. (Both of these processes are being done through the Department of Natural Resources.)

“We want Silver Lake to be a profound success story,” Chu added. “I think a lot of city lakes struggle with these same issues, and we’re on a new frontier where we’re going to recover from the damage that has been done to Silver Lake. We have a goal of making it one of the most diverse and cleanest lakes in Ramsey County.”

Beyond netting the fish and managing aerators, Kocian encouraged anyone interested in helping with water quality to join a lake association if they live on a lake. If you don’t, he suggests considering how your home or garden impacts lakes and streams, whether by reducing fertilizer use, planting a rain garden or keeping leaves and grass out of storm drains.