Water samples from the St. Croix River have tested positive for genetic material from silver carp, suggesting the invasive, leaping Asian species may be present in the river as far north as the dam at St. Croix Falls, according to the Minnesota Department of Natural Resources.
Similar tests in the Upper Mississippi River were inconclusive, so more testing will be done.
Known as environmental DNA testing, eDNA is a new but scientifically accepted method of screening water samples for genetic material originating from an aquatic animal's mucus or excrement. An eDNA report does not provide any information on number of fish present, their size or whether they are breeding, officials said.
To date, no silver carp have been caught in the St. Croix River, either by anglers or commercial fishing operators. Only two bighead carp, a different Asian species, have been caught in the river - one in 1996 and another on April 18 of this year.
The eDNA discovery has prompted the DNR to take several actions.
"Our immediate goal is to mobilize as much effort as possible to confirm the presence of live silver carp in the St. Croix," Commissioner Tom Landwehr. "The results raise the profile and the level of urgency around the Asian carp issue not just for the DNR, but for all agencies, conservation groups, municipalities and river users who have a stake in the health of the St. Croix and the Mississippi."
The DNR will soon contract with commercial fishing operators to begin using nets on the St. Croix to try to capture live silver carp in the same areas where eDNA tests were positive. DNR staff will also use nets and boats outfitted with electric shocking capabilities to search for fish.
DNR operations could start next week; commercial netting operations are expected to start by the end of August.
DNR officials said they will proceed with development of a bubble or sonic barrier at the mouth of the St. Croix River at Prescott, pending results of the additional carp sampling. Scientists believe such a barrier would not be a 100-percent deterrent to Asian carp, but if the populations are low, the barrier could help keep additional carp out of the river while other population control methods are developed.
Testing in June
On June 28, a private contractor collected 50 samples from a 4.3-mile stretch of the St. Croix River. The sample area started at the St. Croix Falls Dam and went downstream to near the town of Franconia. The sample area was roughly 48 to 52 miles upstream of the river's confluence with the Mississippi River at Prescott.
On June 29, the same contractor, Environmental DNA Solutions from Granger, Ind., took 50 samples from the Mississippi River in St. Paul. The samples were collected in a river stretch starting at Lock and Dam 1 and ending 3.6 miles downstream at Pike Island. The DNR received the test results Aug. 4.
All 100 samples from the St. Croix and Mississippi rivers were tested for three species of Asian carp: black, bighead and silver. The three species 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.
All 100 samples tested negative for bighead and black carp. All the samples tested negative for silver carp in the Mississippi River.
Scientists cautioned that the negative results on the Mississippi River do not mean silver carp are not in the river. They said the Mississippi was at flood stage during the testing process, which could have limited the accumulation of carp DNA.
DNR staff note that bighead and silver carp have been caught in the Mississippi downstream of Lake Pepin.
In contrast, the positive samples from the St. Croix River were more conclusive. Twenty-two of 50 samples (44 percent) tested positive for silver carp along the 4.3-mile stretch of river.
Minnesota Gov. Mark Dayton recently signed a bonding bill, approved by the Minnesota Legislature, funding a $16 million upgrade of the Coon Rapids Dam on the Mississippi River. The dam improvements are designed to provide a permanent barrier to the upstream migration of Asian carp to the upper reaches of the Mississippi River.
Construction should begin in 2012.