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Published 4/28/2010

The test that detected Asian carp's environmental DNA, or eDNA, above the electric barrier in the Chicago waterways leading to Lake Michigan, was developed in the past year by New Zealand scientist Lindsay Chadderton and researchers at Notre Dame. They say this is the first time DNA testing has been used on such a scale to find evidence of invasive fish in fresh water, and they think their method will ultimately be used around the globe to detect invasive species and protect endangered ones.

In January 2007, the Nature Conservancy hired Chadderton, an aquatic ecologist, as the first director of aquatic invasive species for its Great Lakes Project, a program to rejuvenate and protect the ecosystem of the lakes. Chadderton had been working with invasive species for New Zealand's Department of Conservation, and soon after he arrived in the Midwest, he and a team from Notre Dame began experimenting with DNA detection. Now this technology has taken center stage in perhaps the most politically charged invasive species controversy ever to hit the Great Lakes region.


Kari Lydersen:
How does the DNA test work?

Lindsay Chadderton: We go out, take between 60 and 100 two-liter water samples, put them in coolers on ice to slow natural breakdown, and bring them back to the laboratory. Within 24 hours of collection, we filter each sample through really fine filter paper ... to remove any particles in the water, including any possible cells. Then we extract the DNA off the filter paper, using kits that break the cells open and release the DNA. We then use a centrifuge to take the liquid off, and that gives us a DNA extract. Then we amplify — sort of clone — the species-specific DNA we're looking for so we have enough to detect, and we run these on simple gels that allow us to compare each sample with controls that contain the target DNA.

Lydersen: The Army Corps of Engineers and state officials have repeatedly stated that no Asian carp have been found above the electric barrier. How confident are you of the DNA tests?

Chadderton: We are confident that our results are real, and the more testing we do, [the more] this confidence increases. In the criminal justice system, we regularly use DNA to place people at the scene of a crime. People, like other animals, shed DNA into the environment — skin, hair, bodily fluids. Carp do the same thing — DNA cells associated with mucus or sloughed off from the gills, attached to scales, shed from the gut system, and contained in feces and urine. Once those cells are released into the water they are held in suspension for some time and we are simply collecting them in the water column. We're looking for evidence of a species, instead of individuals like you do with people, but the principles are the same.

[One] thing that gives us confidence is the fact we can go back to certain places and repeatedly detect DNA. The distribution is consistent with the movement of fish. For example, the number of positive samples decreases as we get closer to the barrier. That's consistent with an upstream invasion.

Lydersen: Why are the Asian carp moving past the barrier at all? They don't know there's good habitat farther on, do they?

Chadderton: It's probably just an innate sense. These fish prefer slow-flowing water and the canal is probably not particularly good habitat for them. It's like a box, there are not a lot of places to get out of the current so we'd expect these fish to continue pushing up through the system searching for better food or habitat. With any population there are dispersers that keep moving and other animals that hang around. For one thing, if you're at the front of the pack there might be more food. If you've got hundreds of thousands of individuals behind you all feeding in the same water, wouldn't you want to be up front?

Lydersen: When fish in the canal were poisoned in December, just south of the electric barrier, just one dead Asian carp was found. Why was that, if there are so many Asian carp in the system?

Chadderton: We didn't expect to see many — if any — Asian carp, as when they are killed by rotenone they sink. It was cold, which makes them slower to decompose and bloat and float. There were apparently large numbers of fish on the bottom being fed on by crayfish and other invertebrates not affected by the fish poison. All these conditions would have acted to prevent the fish floating to the surface.

Lydersen: You've spoken of using a "Judas carp" to find Asian carp already past the barrier and possibly in Lake Michigan. What is it?

Chadderton: We think some fish have gotten into Lake Michigan — it's the most plausible explanation for what we have found. So, if we want to stop Asian carp, we need to track them down and prevent them from spawning. One way would be to use Judas fish. These are fish with a transmitter attached to them. You follow the Judas fish around, and once it's hanging out you encircle the area and fish it out or treat it [with rotenone or another poison]. The technique is used to control invasive species around the world. The Aussies have used it very successfully to control and almost eradicate common carp in two lakes in Tasmania. We need to remove the Asian carp between the barrier and the Great Lakes, and we could use the Judas fish to locate areas to treat.

Lydersen: Do you think we could actually find the probably very few Asian carp in Lake Michigan?

Chadderton: When the carp are in the lake, they have an Achilles' heel — they need long distances of running water to successfully spawn. So while it's going to be hard to find them in the lake itself, the fact they will likely need to go inland and upstream to spawn may make the search easier. There's a lot we can learn from the sea lamprey control program, which is the poster child of integrated aquatic pest management. [The government spends about $18 million a year fighting invasive sea lampreys, which latch on to Great Lakes fish and suck their blood. Fishery managers employ various barriers on Great Lakes streams to block and capture lamprey after they move inland to breed. They also poison the larvae and release thousands of sterile male lampreys to mate with the females.]

Lydersen: Could you apply rotenone to all the canals and rivers leading into the lake?

Chadderton: Treating the whole thing would be something you'd want to explore. There are other methods you could use, like electrofishing or seining. But treating the waterway with a fish poison like rotenone is the only surefire way of making sure you got them all.

Lydersen: Do you still have faith in the electric barrier?

Chadderton: It's a numbers game — the more fish that get into the lake, the greater likelihood we will see successful establishment. We know the largest numbers of fish are still below the barrier and we need to keep them there. Over the last two to three years the upper barrier has been taken down for maintenance on at least one occasion, on the assumption Asian carp weren't present. But Chris [Jerde, from Notre Dame] estimated the carp probably made it to right below the barrier two to three years ago, and it seems possible that during the 2008 maintenance operations fish got through.

Lydersen: You are pretty certain Asian carp are in Lake Michigan. Does that mean the worst fears of environmental groups — that the lake will be taken over by Asian carp — will come true?

Chadderton: It's definitely not game over. We've got some amount of time. There's got to be enough fish, they've got to find each other, they've got to find suitable spawning habitat, their eggs have to survive and hatch, the larvae have to survive. At each stage all sorts of things could go wrong, there's still lots of uncertainty.

If we look at invasion history around the U.S. and the world, invasive species often don't do what we expect. They may not end up being more than a nuisance, but they could conversely end up being a disaster. We won't know until it happens, at which stage it's too late. It seems incredibly risky or foolhardy to let these things into the Great Lakes unchecked. We know enough about them to be really concerned about the potential consequences. There is too much at stake. And we never really know what exactly is going to happen until it happens.

Lydersen: Since Lake Michigan has relatively sparse plankton and Asian carp need faster-moving water to spawn, it doesn't sound like it's even an ideal habitat for them.

Chadderton: In their native range, they prefer slow-flowing or standing water, like lakes — except for when spawning. The big issue is whether there is enough food. This is likely to vary across each of the Great Lakes, and some recent work suggests that in the open water of most lakes there isn't enough food. But that research work didn't take into account all possible food sources — like the larvae of [invasive] zebra and quagga mussels. Everybody generally accepts that Asian carp are probably likely to do well in Lake Erie and in embayments in Lake Michigan like Green Bay.

Lydersen: They've already been found in Lake Erie, right?

Chadderton: About five bighead carp have been found in Lake Erie over the last 14 or so years. The assumption is these fish represent individual releases. At least two of them were in really good condition — they were fat and large.


Kari Lydersen is a Chicago-based freelancer, author and journalism instructor. Her latest book Revolt on Goose Island (Melville House) chronicled the 2008 Republic Windows factory occupation. A longer version of this interview originally appeared in Chicago Reader.

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