Kansas City, Mo. ? In just four years since the first zebra mussels were found in El Dorado Reservoir near Wichita, Kan., the invasive species has multiplied so rapidly that the mussels are becoming a safety hazard to swimmers and a potentially costly nuisance for marinas and boat owners alike.

“They’ve pretty much covered our entire underwater habitat at El Dorado, and they’re very dense,” said Jason Goeckler, aquatic nuisance specialist for the Kansas Department of Wildlife and Parks.

At El Dorado, the mussels have formed dense colonies on underwater rocks and trees. It is believed that they entered the reservoir when attached to boat hulls or floating in live wells, bait buckets and bilge pumps.

Biologists are encouraging boaters to take precautions such as thoroughly cleaning boats and trailers before moving between bodies of water. They also encourage swimmers to wear foot protection and fishermen to be prepared for some damaged fishing line.

“If you’re swimming or wading and you don’t wear protective footwear, you’ll cut your feet,” Goeckler said. “If you’re fishing near the bottom after five casts or so, your line will be cut. Some people are starting to use steel leaders.”

Zebra mussels have caused billions of dollars in damage in the Great Lakes and Mississippi River basins. They are the size of a BB when they first form shells and attach to a surface, and eventually grow to about thumb size.

Goeckler said one liter of water from the lake averages 300 of the mussels’ larvae, and biologists are finding an average of 65,000 mature mussels per square meter.

He said the lake still is safe for visitors, but they need to take precautions.

Zebra mussels also were found last June in an arm of the Lake of the Ozarks and are now being found throughout the lake in low densities. Signs are posted at most boat ramps in the region encouraging boaters to clean and dry all gear before moving to a different lake or river.

“We’re very concerned about the possibility of accidentally transferring them into other bodies of water,” said Brian Canaday, invasive species coordinator for the Missouri Department of Conservation.

Scientists have not found a way to eliminate mussels from large lakes or rivers after they have become established.