SIOUX CITY | A puzzling case of hundreds of dead Asian carp found floating in the Missouri River has been eating at fisheries biologist Jeff Schuckman for weeks.
The mystery revealed itself in May, when the decaying fish carcasses were found stinking up the river’s slow backwaters near Vermillion, S.D., and Ponca, Neb.
All but a few of the dead fish were silver carp, an invasive species.
Two months later, local fisheries biologists and conservation officials still don’t know what killed them. Though the fish are unwanted pests that ravage the ecosystem, experts fear whatever killed them -- whether disease or environmental factors -- could affect other fish. The answer could also be key in how to get rid of Asian carp, which are aggressively competing for food with native fish.
Asian carp are also turning up dead in Missouri. Some of the aquatic pests are also believed to have died in Nebraska’s North Platte River in May, when boaters reported seeing them floating near the mouth of the waterway in the Missouri River south of Omaha.
So far, no one has come up with any answers.
"I don't know what killed them, Schuckman, of the Nebraska Game Fish and Parks Commission, said.
ENVIRONMENTAL, SAFETY THREAT
To many, dead Asian carp is a good thing. The more, the better.
Three varieties of carp -- silver, bighead and grass -- are in the Missouri River near Sioux City. The fish grow quickly, gobbling up plankton needed by native fish and disrupting the food chain.
All three species of carp were imported from Asia to clean sewage ponds and fish farming facilities in the southern United States. They escaped during high water and migrated north.
Fisheries biologists and conservation officials tracking their advance up the Mississippi River, the Missouri and tributaries have long feared the ravenous eaters’ ability to take over the ecosystem. Their numbers swelled after the record Missouri River flood in 2011.
Asian carp also pose a safety hazard. When startled, silver carp hurl their bodies from the water into the paths of boaters.
In May, the same time silver carp were found dead in Siouxland, anglers and boaters spotted other dead Asian carp floating in the Missouri River south of Omaha. Officials don’t know for sure but believe those fish came from the Platte River.
Asian carp are still dying in the Missouri River in the state of Missouri. Officials think those fish are starving.
"Fish that should have been 20 pounds due to their length were about 12 pounds, and I’ve seen them even worse than that," said Duane Chapman, a U.S. Geological Survey biologist based in Columbia, Mo. "They're extremely emaciated."
Officials don’t think the carp that died on the upper Missouri starved. They weren’t thin, and native fish appear to be in good condition.
Experts also don't think environmental factors were at play. If the silver carp died because the water was too warm, too low or didn’t have enough oxygen, all potential fish-kill culprits, other fish would have died as well, Schuckman said.
"What I observed was a single-species die-off, and that makes me think it was something else involved," he said.
It's possible the fish may have died of disease, but officials weren't able to take any samples to find out. They need dead or dying fish for testing, something they didn’t get from the Vermillion and Ponca fish kills.
Anglers, boaters and river enthusiasts can help by calling authorities if they see a large number of dead fish.
Officials say whatever’s killing the Asian carp may be something that could help control their population, but it’s too soon to know. So far, the fish have evaded control efforts.
Once kept from spreading by a dam on the Little Sioux River, the fish swam over it during the 2011 flood. They went up the river and into the Iowa Great Lakes.
Great Lakes officials, with help from Minnesota, have since built a nearly $1 million electronic fish barrier across the Lower Gar Lake dam. They can’t get rid of the Asian carp already in the lake but hope the barrier deters more from entering the popular recreational lake chain.
If the silver carp found dead in May died of disease, it’s possible more outbreaks could occur. Even if it does, wildlife officials say such a disease is a double-edged sword and are loath to promote spreading disease as a way to kill the fish.
While it may be effective in killing silver carp, it could also mutate and infect other species.
That’s why officials haven’t used the koi herpes virus, responsible for several common carp die-offs, to control the population of that undesirable fish.
Hundreds of thousands of common carp were killed in recent years by an outbreak of the virus. An estimated 40 tons of carp were killed in the Calamus Reservoir in north-central Nebraska in 2010. Another outbreak was reported in Lewis and Clark Lake, near Yankton, S.D., in 2012.
Even the deaths of undesirable species are important to the overall health of the river, officials say.
“A lot of people think, 'Oh, it’s just Asian carp,' or, 'It’s just common carp' that are dying, but that’s a big deal to us,” Schuckman said.