Few if any Asian carp in St. Croix, Mississippi rivers

Asian carp
From left to right: Commercial fishermen George Richtman, Tim Adams and Bob Davis hold Asian carp caught March 1, 2012 in the Mississippi River. Richtman is holding a grass carp, Adams is holding a silver carp and Davis is holding a bighead carp.
Photo courtesy Nicholas Schlesser, MN DNR

Researchers say they have found little evidence of bighead and silver carp in the Mississippi and St. Croix rivers.

The latest research on Asian carp in the Mississippi and St. Croix rivers shows a small number of the invasive fish, but probably not a reproducing population. The research released on Thursday indicates that the invasive fish have not become established in Minnesota.

However, experts say the fish still pose an immediate threat in the two rivers.

Tests last year, using improved techniques to identify the DNA of Asian carp in Minnesota waters, seem to contradict earlier studies which found evidence of the fish in numerous locations in the Mississippi, including above the Coon Rapids dam north of the Twin Cities.

Researchers have two basic ways to try to assess the presence and abundance of Asian carp: measure the carp DNA in the water, or catch the fish themselves.

But last year, researchers added DNA sequencing as a step that they say makes the results more reliable. They found no DNA evidence of carp in the Mississippi or the St. Croix.

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Research detected silver carp in Iowa where the fish are abundant, but detected no silver carp eDNA in key locations in Minneapolis. The U.S. Army Corps of Engineers tested the technology in the Illinois River and Great Lakes, using DNA sequencing as a final verification step.

Asian carp
Silver carp in a tank Thursday, Apr. 4, 2013 at the Minnesota Aquatic Invasive Species Research Center on the University of Minnesota St. Paul campus.
MPR Photo/Jennifer Simonson

"The 2012 analysis was much more rigorous," said Peter Sorensen, director of the Minnesota Aquatic Invasive Species Research Center. "It used a step called sequencing, which actually isolated the DNA and looked at the chemical code to a get precise fingerprint. So we have a great deal of confidence in that."

On the other hand, the samples did not show bighead carp in Iowa, although the fish are known to be found there. No bighead carp were detected in Minnesota either. There's more work to be done to make the tests more accurate, Sorensen said.

Sorensen said there's no denying that carp are in Minnesota, because commercial fishermen routinely pull a few of the fish from out of their nets. While that is convincing evidence, it cannot tell us how many fish are there, he said.

"These are still very smart, fast fish, they avoid nets, they avoid electro-fishing gear," Sorensen said. "We know the netting's very inefficient."

The DNA technique can work together with catching the fish, because workers can sample for DNA in parts of a river where nets would not be effective. Additionally, researchers hope eventually eDNA techniques will provide more information.

So far the tests are very basic, said Jessica Eichmiller, a post-doctoral student working with Sorensen.

"The method that we've been talking (about) all along is positive or negative: detect or non-detect," Eichmiller said.

Eichmiller wants to be able to determine from the samples how many of the target fish are present. She's learning how environmental conditions, such as temperature, can affect testing success.

"At higher temperatures, a lot of times DNA degrades more quickly, so you might have more sensitive detection at cooler weathers," Eichmiller said. "Basically you'll want to combine what we learn about environmental factors to get the most sensitive level of detection."

That level of fine-tuning is in the future.

The latest research confirms what is already known, said Steve Hirsch, director of the Department of Natural Resources Ecological and Water Resources division.

"Which is, that we've got Asian carp. We're periodically catching them. We don't think they're established yet," Hirsch said. "We don't think they're reproducing yet, but we're very worried that someday they will be."

The DNR is hiring an engineering firm to design an electric barrier to use at the Ford Dam. It is a new technology and it's unknown whether it will work. Hirsch said the most effective tactic would be to close the lock.

"Closing the lock could be 100 percent effective, at least in terms of stopping the upstream migration of the fish," he said.

A bill in Congress would require the U.S. Army Corps of Engineers to close the Upper St. Anthony Falls lock, depending on how close the fish get.