Zebra mussels take last of area's recreational lakes
SPIRIT LAKE — Center Lake is one of the latest in the state to have a confirmed case of zebra mussels. Mike Hawkins, fisheries biologist with the Iowa Department of Natural Resources, said about 20 juveniles were found the first weekend in August on three monitoring plates the DNR hangs beneath navigational buoys.
"Our watershed goes into Minnesota, and we don't have confirmation on Little Spirit or Loon Lake from Minnesota," Hawkins said. "In the lower chain here — the Big Spirit-Okoboji chain — this is kind of our last water body out there. There's some smaller shallow lakes in the watershed like Welch Lake, Pleasant Lake and Prairie Lake. Those do not have zebra mussels."
Hawkins said Center Lake was the last of the Iowa Great Lakes' major recreational lakes free of the invasive species, and the DNR expects the mussels to grow substantially in population over the next three to five years.
"They're ecosystem engineers, so they have a way of really upsetting the applecart," Hawkins said.
Aside from the scrapes and cuts a zebra mussel's ridged shell can give swimmers, humans aren't greatly impacted, according to Hawkins. But he said, while beach goers can counter the nuisance with proper footwear, fish and other species of aquatic life don't have it so easy.
The mussels can greatly disrupt the availability of nutrients in the water, according to Hawkins.
"The adults can filter up to a quart of water a day, and they can reach densities of 10,000 individuals per square yard," Hawkins said. "As you can imagine, they're filtering a lot of plankton out of the bottom of the water. They digest and excrete that, and it ends up on the bottom of the lake."
The lack of plankton is particularly concerning for larval fish which depend on it for food, Hawkins said. He said the tiny striped mussels can even be a detriment to walleye stocking if conditions are right. Fortunately, he said Center Lake is more often used for stocking walleye fingerlings — more adolescent fish — which don't depend as heavily on plankton. On the flip side, he said pan fish and bass generally see more success when sharing waters with the invasive species.
But once the sharp-shelled invaders settle in, there's little getting rid of them.
"There's been a little bit of work done in Minnesota on trying to treat isolated areas of the lake once it's been infested, but that actually hasn't worked for them," Hawkins said. "By the time they can get a treatment done, the thought is that they've spread beyond that treatment area by that point."
Hawkins reiterated juvenile mussels were found in several areas of Center Lake, indicating the population is spread across the body of water. The DNR will confirm the spread later this fall as boat hoists and docks come out of the lake. The mussels are known to cling to such surfaces as well as boats and other aquatic machinery.
At this point, Hawkins said the concern is preventing the mussels from spreading to other bodies of water. Locally, he highlighted Lost Island Lake, Trumbell Lake, Five Island Lake and Silver Lake, but said the public should take steps to keep invasive species out of any body of water near or far.
"It's always the same — clean, drain and dry," Hawkins said, reciting the DNR's campaign slogan. "That's not only for boats, but also hoists, watercraft lifts and docks. We think there's a good possibility these organisms are being transported on those even more readily than via boat and trailer."