Adult zebra mussels have been spotted in Lake Francis Case, South Dakota Department of Game, Fish and Parks officials confirmed on Wednesday.

The reservoir, which spans 102,000 acres from Big Bend Dam at Fort Thompson to the Fort Randall Dam at Pickstown, marks the third along the Missouri River to be deemed infested with the invasive species, which are known for multiplying at a rate that makes them nearly impossible to eradicate.

“The invasive mussels were found on plate samplers attached to docks and also along shoreline areas,” said Chamberlain Area Fisheries Supervisor Chris Longhenry said in a statement. “Densities are low in Lake Francis Case at this time, but the mussels are widespread and present through most of the reservoir.”

In July, zebra mussels were confirmed in Lake Sharpe, another mainstem Missouri River reservoir located upstream from Lake Francis Case. Reproducing populations of zebra mussels were discovered in Lewis and Clark Lake and in the Missouri River below Gavins Point Dam in 2015.

In a Daily Republic story published earlier this month, GF&P officials said they feared that there was a high probability that zebra mussels were in Lake Francis Case but that hadn't been confirmed. GF&P met with area stakeholders in Oacoma and Pierre last month about creating a plan to battle the invasive species, and how the state plans on upping its response to zebra mussels.

Waters infested with zebra mussels are designated as containment waters. Specific decontamination requirements exist for boats kept in these waters continuously for three or more days, or that cannot have all water drained from them.

“Boaters with watercraft that fall into these categories need to learn how to properly decontaminate their boats to slow the spread of zebra mussels,” said Longhenry.

GF&P advised that boaters and anglers should clean watercraft and trailers of all aquatic plants and mud, drain all water by removing all drains, plugs, bailers, or valves that retain water, and dispose of unwanted bait in trash or fish cleaning stations when leaving the water.

GF&P said those steps need to be taken every time a boat is in the water, and "completely draining a boat is the first step in making sure invasive species are not transferred to other waters."