The test that detected the Asian carp’s environmental DNA, or eDNA, above the electric barrier is new: it was developed in the past year by New Zealand scientist Lindsay Chadderton and scientists 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 freshwater, and they think their method will ultimately be used around the globe to detect invasive species and protect endangered ones.
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About 18 months ago I had a conversation with Andy Mahon and said, “What do you reckon?” about trying to see if we could detect DNA in the water column. He said, “Let’s give it a go.” So Andy, Chris Jerde, and I set out doing some really simple trials, putting fish in buckets, seeing if we could detect DNA. Once we saw we could do that, we went out to a local river and found we could detect common carp in the river. At the same time we were playing around with this technique, Chris was also doing a modeling study of Asian carp in the Sanitary and Ship Canal. He thought the fish should be much higher up [the canal] than current methods were saying they were. So last April we went out with the Army Corps and took water samples in the Illinois River where large numbers of bighead and silver carp were known to be present. The day we went out the river was in flood, running five foot above normal river height, and as we drove home with our samples Andy Mahon and I looked at each and said, “Well, it was a nice idea.” We were highly skeptical that we would detect anything because the river at that point is really large, maybe up to 200 meters wide and 10 meters deep, and it was flowing really quickly. We thought any DNA would be flushed out of the system. Hence, when we found we could detect DNA even in small [500 milliliter] water samples, we were really surprised. We then started sampling moving up the Des Plaines River into areas where few Asian carp had ever been recorded, and into areas supposedly above the invasion front, to see if our detection methods were more sensitive than standard electric fishing and static nets. Chris Jerde’s dispersal modeling suggested the fish should have made it at least to the electric barriers designed to prevent the carp getting upstream, so we were not surprised to find evidence of fish. Once we found DNA below the barrier and coupled that with some dispersal modeling done by Chris, a math biologist, we were also not that surprised we found DNA above the barrier. We did the first trials in April 2009 and the first tests in June.
How does the DNA test work?
And the last thing that gives us confidence is the fact we can go back to certain places and repeatedly detect DNA. These results are not chance events, and 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.
You’ve spoken of using a “Judas carp” to find Asian carp already past the barrier and possibly in Lake Michigan. What is it?
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.
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.
