YANKTON, S.D. - Aliens have landed at the mouth of the James River, and while they've been friendly so far, there's a good chance they'll turn hostile.

Two species of Asian carp - the bighead and silver carp - have been menacing native fish at the southern end of the Missouri River for years, but in the last few years they've traveled north and now threaten the area's native species, according to Wayne Stancill, fisheries supervisor for the U.S. Fish and Wildlife Service.

"It wasn't until two years ago that we saw the first silver carp, and that was in the James River," he said. "I think they are definitely becoming more abundant."

The James River runs 710 miles through the Dakotas.

"They're here. There's no doubt about it, they're here," said Don Lepp, a landowner at the mouth of the James River. "There were a lot of them all up the Jim River last summer."

In fact, the silver carp - renowned jumpers often able to heft up to 40 pounds as far as 10 feet in the air - are now so abundant they are posing a safety hazard to all motor boaters on the river below Gavins Point Dam.

Another dire problem is that the Asian carp are so competitive in the river systems that they drive out native species. This could have an extremely bad effect on game fish and more desirable species.

But for right now, the most noticeable impact seems to be their acrobatics.

"One of the things we learned early on with these fish is that they are extremely sensitive to disturbance," said Stancill. "(While) the silvers jump, the bigheads just move out of the area. I've heard that going down the Vermillion River is now pretty dangerous if you're going down through there in a motorboat."

"The silver carp response to disturbance is jumping. They are extremely strong jumpers."

"(Stancill was) working on the James River with another guy, we were setting up some larval nets tied to the front of the boat. We were attempting to determine if the fish were spawning. One of the fish ran into one of our submerged nets, and when he hit that net he jumped into the air and came right into the boat behind me. This fish weighed 20-plus pounds. If he had come down a half a foot closer, he may have hit me in the head and knocked me out of the boat."

In fact, the Silver carp are so sensitive to disturbance, that Stancill and his fellow researchers were forced to modify their measurement techniques.

"We were watching some Asian carp and we noticed that about half a mile up the river, a boat was coming toward us," he said. "At about a quarter-mile away, they were able to detect the disturbance and left the water column."

Right now, researchers below the dam are mainly concerned with whether the Asian carp are here to stay or if they're just passing through.

"Typically when a fish starts showing up in a new area or pioneering, they will be mostly adults," said Stancill. "We start looking for younger fish. Right now we haven't seen any, so we're not sure if what we're seeing are just pioneering adults."

The best-case scenario would be for the current fish to be pioneers and continue on their way up the tributaries to another distant home to spawn. From what Stancill and other biologists have researched, the fish like to spawn at tributary mouths when the water is higher than normal. So if the James and Vermillion Rivers remain low there is a chance these fish could be passing through.

But according to Stancill, there is little hope for them deciding to move out.

"There's really no reason for us to think that they wouldn't spawn but we haven't seen it yet," he said.

The most common story about how the silver and bighead carp escaped into the Mississippi River, and subsequently ended up below Gavins Point Dam, is that they were originally used to control weeds and algae in fish farm ponds in the south.

"They were used for controlling phytoplankton in aquaculture facilities," said Stancill. "In some instances they were actually brought in as a food source."

When the great floods of the early 1990s caused the farm ponds to overflow and join adjacent rivers, the fish were free to expand and flourish devoid of predators in an unlimited network of rivers.

"We've always thought that these dams were a barrier to upstream migration. There's been a lot of discussion among the biologists and with the Corps about providing upstream passage," said Stancill. "Right now there's a lot less support for providing fish bypass above these dams now primarily because of the concerns about moving these Asian carp up into the reservoirs. It has almost brought the discussion of fish passage to a halt."

For right now, Don Lepp and other landowners along the James River still consider the carp to be natural entertainment, but if injuries start resulting from boaters running in with fish, that sentiment could change.

"It's still a novelty right now," he said. "It seems like they're fitting in quite well and will do well for themselves."