Water samples from the Mississippi River downstream from the Ford Dam in Minneapolis have tested positive for genetic material from silver carp, indicating the invasive Asian species may be present in the Twin Cities stretch of the river, according to the Minnesota Department of Natural Resources.
Known as environmental DNA (eDNA) testing, the results are a chemical indication that some silver carp are in the river, but they do not provide any information on the possible number of fish present, their size or whether they are breeding.
The Mississippi River eDNA testing was conducted in September by the National Park Service and the DNR after similar testing in June indicated the presence of silver carp in the St. Croix River.
The DNR will immediately hire a commercial fisherman to begin netting and searching for Asian carp below the Ford Dam, also known as Lock and Dam 1. No Asian carp were discovered this summer in the St. Croix River after a nine-day search by DNR biologists and a commercial fisherman, but that doesn't necessarily mean some fish aren't present.
"The eDNA tests are very sensitive, but they can only tell us that DNA is present in the water," said Tim Schlagenhaft, Mississippi River biologist. "In other states where DNA testing has resulted in positive samples, the fish have proven very difficult to subsequently capture, and we expect this to the case in the Mississippi River if the fish are in present in low numbers."
In the most recent round of Mississippi River eDNA testing, 14 of 49 samples were positive for silver carp. The samples were among 275 that were collected in September from the Mississippi, Minnesota and St. Croix rivers. The rivers are being tested for two species of Asian carp: bighead and silver. The DNR is awaiting test results from the other river locations.
National Park Service officials said these new results could mean Asian carp are present in a reach of the river that includes the Mississippi National River and Recreation Area, a unit of the National Park System that runs along 72 miles of the Mississippi River in the Twin Cities.
"These eDNA results are like a smoke alarm blaring. Until we find the source, we have to assume there is a fire. We have to assume Asian carp are here," said park superintendent Paul Labovitz.
Labovitz said the park officials may discontinue using Mississippi River locks when conducting NPS-sponsored canoe and boat programs that introduce school kids to the river. When trips involve opening river locks, Asian carp may be able to move freely up and down the river. "We must explore every option to slow down these fish," Labovitz said.
DNR officials said the positive eDNA tests highlight the importance of taking further actions to protect upper reaches of the Mississippi River from Asian carp. Gov. Dayton recently endorsed a seven-step action plan to address an invasion of Asian carp.
The plan supports immediate congressional action to give the Army Corps of Engineers emergency authority to close the Upper St. Anthony Falls Lock, and Lock and Dam #1, if Asian carp are detected nearby. It also asks for immediate congressional action to fund a feasibility study examining the economic, ecological, recreational, legal, and operational impacts of making Upper St. Anthony Falls Lock a permanent fish barrier.
"We are committed to getting all government agencies and elected officials engaged in this effort to address Asian carp in our rivers before we have a large, reproducing population," said DNR Commissioner Tom Landwehr.
To date, no silver carp have been caught in the St. Croix or Mississippi rivers above Lake Pepin, though some have been caught further downstream near Iowa. Only two bighead carp have been caught in this area - one in 1996 and another on April 18 of this year - both in the St. Croix River.
DNR fisheries experts say silver and big head carp are extremely skittish and difficult to catch with traditional netting and electroshocking equipment. The two species of carp could cause serious damage to Minnesota's native fish and aquatic systems by filter-feeding vast amounts of plankton, a key foundation of a river's ecosystem and food chain.
eDNA testing is a new but scientifically accepted method of screening water samples for genetic material originating from an aquatic animal's mucus or excrement.