Lakes Superior and Erie have too many sea lampreys.
The invasive fish latch onto big fish like lake trout and salmon and drink their blood and body fluids. A single lamprey can kill up to 40 pounds of fish in its lifetime.
Marc Gaden is the communications director for the Great Lakes Fishery Commission. It's the commission’s job to keep the sea lamprey under control -- a job that costs about $21 million a year.
Gaden's group reported this month that lampreys are at historic lows in Lakes Michigan, Ontario and Huron.
But lamprey numbers have increased in Superior and Erie.
Gaden estimates that there are 80,000 lamprey too many in Lake Superior.
"Why are they so high in Lake Superior? We don't know for sure, but we think that it could be a rebound; lampreys rebounding from a couple of very harsh winters," says Gaden.
"You might remember a few years ago we had some harsh winters downstate, and it certainly was the case in the northern parts of the Great Lakes region, as well, followed by a few milder winters, and maybe the lamprey took advantage of that and rebounded."
Gaden adds that there could also be more food in the lake, and therefore more fish for lamprey to prey upon.
And there could be new ways lampreys are getting into the lake.
"One area that we're very concerned about, and we're keeping a very close eye on, is the St. Louis River. It's a very large river at the western end of Lake Superior near the border between Minnesota and Wisconsin. It didn't produce lamprey, it's been polluted, and they've spent a lot of time cleaning up that river system. And it could be in the future a lamprey producer. We don't think it is now, but those are the kinds of things we keep our eye on," he says.
Gaden says in Lake Erie, lamprey levels are three times above the target level (there are about 9,000 too many lampreys). He says although the number of lampreys is on an overall downturn, there's been an uptick in recent years.
"The problem with Lake Erie is lamprey, we think, are coming from outside of the lake. So we've treated all the streams in Lake Erie that have lamprey in them - there's not many. We treat them again, and we treat them again, and we go out there, and we assess it, and sure enough: there aren't many lamprey left in those streams. But yet the numbers are very high in Lake Erie," he says.
The St. Clair River, Lake St. Clair, and the Detroit River are all suspected sources of the lampreys in Lake Erie.
But there could be more money heading toward the fight against sea lampreys. Gaden says the U.S. Senate is proposing about $7 million more in lamprey control.
He says after meeting with Republicans and Democrats from the Great Lakes congressional delegation, he is cautiously optimistic that the money will come through.
"There's a great amount of enthusiasm for the possibility here. I can tell you that our congressional delegation fights very hard for invasive species funds," he says.