There's renewed concern over the approach of Asian carp into Minnesota waterways, prompted by environmental DNA tests that showed traces of the species' DNA in several locations in the Mississippi River in the Twin Cities. Two species of the carp, bighead and silver carp, are considered invasive species and a threat to the Great Lakes.
The Department of Natural Resources and commercial fishermen have been searching for the fish in Minnesota waters. The fish are known for their immense size and appetite: They can weigh up to 110 pounds and can consume up to 20 percent of their bodyweight each day in food -- mostly plankton. They've alarmed fisherman in other states by jumping several feet into the air and landing inside boats.
"We cannot catch the fish right now, so that tells you that the numbers are very, very low," said Luke Skinner, the invasive species program supervisor at the DNR. "There's a very small number of fish in the river, and it's like trying to find a needle in a haystack."
Skinner discussed the DNR's efforts with MPR's Tom Crann this week. An edited transcript of the interview is below.
Tom Crann: So, a lot of talk about this DNA or e-DNA, environmental DNA of the fish present now in the Mississippi right in the Twin Cities metro. What does that mean for us?
Luke Skinner: Well, all fish produce or slough off DNA into the water, whether it's from mucus or excrement. And so what we can do then is use this environmental DNA test to actually see if they're in the river. So they can filter the water. They can find samples, but it's a very, very sensitive test. So it doesn't tell us how many fish, but it does tell us that there is (Asian carp) present very locally to where this test was taken.
Crann: So they're here.
Skinner: At low numbers, potentially. All of our sampling we've done with commercial fishermen and with our DNR staff out in the boats doing netting and testing, you know, we cannot catch the fish right now. So that tells you that the numbers are very, very low. There's a very small number of fish in the river, and it's like trying to find a needle in a haystack.
Crann: How far have these fish traveled?
Skinner: These fish started off in Arkansas, and so they were in fishponds in Arkansas. With flooding and other things, they've escaped into our rivers, and they've been working their way up for a number of years, and we've have them in Minnesota, at least the lower reaches near the Iowa border for the last decade.
Crann: If they've been that close for the last decade, what has prevented them from getting here sooner?
Skinner: Typically what we find is individual fish. So you've got sentinel (fish), so to speak, shooting up the river. They get there. There's not enough fish for them to reproduce and start a population.
Back in April they did catch an Asian carp in the St. Croix River. A commercial fisherman caught one. And again, it was a single lone fish, a big male, adult fish, so it's not likely reproducing, but it's come up the river on some sort of journey, but that's what we're trying to find out, how well established they are and what we can do to help prevent their spread.
Crann: What has worked to prevent their spread so far?
Skinner: The number one way to prevent their spread is with barriers. And that means a physical barrier in the river to keep them from spreading.
Crann: When we talk about barriers, what are we talking about? A physical sort of filter that would catch the fish? Or are we talking about, I've heard about electrical barriers. They shock the fish. How does that work?
Skinner: Well there's several types of barriers. One is a physical barrier. One that they just cannot jump over. And that's what we're talking about here in Minnesota. We're talking about building up and refurbishing the Coon Rapids dam so it'd make it difficult for the fish to be able to jump over it.
In Chicago they're using a behavioral barrier. They're using electrical charge across the river, so when they get in there, they get zapped and they want to retreat. Other types of barriers we're looking at are sound and bubble barriers. Again, it's the behavioral barriers that they don't like to cross, but those aren't fool proof because if they do get through there, they can continue on. So it's not that physical barrier. There's less chance of them getting over a physical barrier than they would some sort of behavioral barrier.
Crann: But the physical barrier, how practical is that, especially with shipping traffic?
Skinner: Well, in a lock and dam system, it's not very practical. Most of our locks and dams on the river, not only can they lock through, but they have roller gates on them to help control the water. So the past couple years when the water's been very high in the Mississippi River, the gates have been open to allow water to rush through because we don't want to back up water. And that allows the fish just to move unimpeded upstream right up to here in this area. So you need a physical barrier that they cannot get beyond.
Crann: From what we see and hear about them, they are big, ugly fish, and they're voracious fish, but what about them is so troublesome that if you look ahead five or ten years and nothing is done, what are we in for?
Skinner: Well it's going to be difficult to tell, but one of our worst case scenarios would be that they get all the way up to the Mississippi River, they get into a lot of the tributary rivers that come into the Mississippi up into central Minnesota, and they impact our fisheries and start to degrade our systems by impacting that base of the food chain and impacting the whole ecology.
And just its sheer numbers, they get very large numbers. In the Illinois River, for example, they get up to 90 percent of the biomass in certain stretches of the river. They just out-compete all of the other fish species.
In addition, the silver carp are the jumping species, and we've seen it where they jump out of the water ... and when they do that they hit anglers or recreational people recreating on the water, and that's of concern, but our number one concern right now is the ecology and the health of the river.
(Interview edited and transcribed by MPR reporter Madeleine Baran)