Red Lake, by far Minnesota’s largest inland lake, was declared infested Monday after zebra mussel larvae were located in troubling numbers during monitoring.

The discovery in Upper Red Lake prompted a state Department of Natural Resources (DNR) expert to concede that adult reproduction there is a near certainty.

Until this confirmation, the combined Upper and Lower Red lakes had been the last of the state’s major walleye lakes to avoid infestation by a significant invasive species, the DNR said.

The sobering discovery in northern Minnesota was confirmed by the DNR after the study of samples gathered last summer by biologists from the Red Lake Nation. While adult zebra mussels have yet to be identified in Red Lake, larvae typically indicate the presence of a reproducing population.

“Unfortunately, the most reasonable conclusion is that they came from adult reproduction within the lake itself,” said DNR research scientist Gary Montz.

Because Red Lake is “a unique lake system — very large and shallow,” Montz said, “it is not possible to estimate the abundance or distribution of zebra mussels in Upper Red Lake from this sample.”

The DNR has been working with the Red Lake Nation for more than 10 years to monitor zebra mussel larvae in the lake and will coordinate with the tribe on how to respond to this latest development.

Zebra mussels are a concern because they can compete with native species for food and habitat, cut the feet of swimmers, reduce the performance of boat motors and cause expensive damage to water intake pipes.

The discovery from the middle of the Upper Red Lake was of eight larvae, “which is a lot,” said Heidi Wolf, a DNR invasive species unit supervisor. “If we just found one, we might not be on the same trajectory.”

Wolf added that the species most likely was introduced to the lake by some human action, such as bringing an infested boat into the water.

Even though the discovery occurred only in the Upper Red Lake, the infestation declaration also covers the Lower Red Lake because “they are so heavily connected,” Wolf said.

Additional waters connected to Red Lake may be added to the infested-waters list after further DNR review, the agency said.

The Upper and Lower Red lakes combine for nearly 290,000 in total acres, more than twice the size of Lake Mille Lacs, the state’s second-largest inland body of water.

DNR and tribal response likely will include continued monitoring, increased watercraft inspections in the area and additional public information efforts, the state agency said.

Whether or not a body of water is infested, state law requires boaters and anglers to clean watercraft and trailers of aquatic plants and prohibited invasive species, drain all water by removing plugs and keeping them out during transport, and dispose of unwanted bait in the trash.

The DNR also cautions that zebra mussels larvae and other invasive species are especially small and difficult to see. To remove or kill, these precautions are best before moving a boat from one body of water to another: Spray with high-pressure water and rinse with water 120 degrees for at least 2 minutes or 140 degrees for at least 10 seconds), then dry for at least five days.

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