Asian carp might have entered lakes, but so what?

Asian carp might have entered lakes, but so what?

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Silver carp, one of the several species collectively referred to as Asian carp. (Michigan Sea Grant/Dan O'Keefe)

New evidence suggesting Asian carp may already be in the Great Lakes basin has renewed fears that the invasive species could pose an existential threat to the area’s lucrative fishing industry.

“The fact that fish may already be in the lake does not mean it’s game over,” said Lindsay Chadderton, aquatic invasive species director for The Nature Conservancy. “The real risk is that if we continue to debate and don’t act, we may lose that opportunity.”

But the charismatic fish, infamous for their tendency to leap out of the water (though they’re unlikely to do so in the deep waters of Lake Michigan), are no shoe-in when it comes to colonizing the Great Lakes.

“In my view, the Mississippi River basin is the least of Lake Michigan’s worries, because the habitat is so warm, rich and shallow that its denizens would be completely unfit in cold, dilute, deep Lake Michigan,” said Russell Cuhel, a senior scientist with the Great Lakes WATER Institute. To reproduce, carp need access to rivers where there is an amply flowing water column to help disperse their eggs. That isn’t common in most of the Great Lakes, but some places, including Lake Erie and the Detroit River, could provide the right conditions.

The sea lamprey, another invasive species that decimated the Lake Trout population, shares an Achilles heel with Asian carp. Like the carp, lamprey head upstream to breed. To control their spread, the Great Lakes Fishery Commission applies specialized poisons that kill young lampreys in streambeds before they reach open water and mature. It’s possible that if carp do establish themselves in the Great Lakes, a similar strategy could control their population. But it’s no sure bet.

“Invasive species never do what we expect them to do,” Chadderton said. “They’re opportunistic. That’s why they’re good invaders.”

At any rate the jury is still out on whether carp could flourish in the unfamiliar Great Lakes ecosystem. Unlike carp, the wildly successful quagga and zebra mussels, which first arrived as stowaways in ship ballast tanks, breed where they live and are capable of producing 1 million eggs per year. In many areas of the Great Lakes they now blanket the lake floor, and have become by far the most dominant species by biomass in Lake Michigan.

Those mussels have devoured much of the available phytoplankton — the same food source carp depend on — posing another challenge for the new invader. Research suggests that carp might be able to survive on other food sources, however, including mussel feces. And even minor competition from the voracious carp, which can eat up to one fifth of their body weight in plankton each day, could place further pressure on young walleye and other sport fish that also eat plankton in their larval stage.

While the lamprey and the equally disruptive alewife entered the Lakes on their own volition, they are the exception to the rule. Recent research published by Cuhel and Carmen Aguilar in the Annual Review of Marine Science found few of the many invaders since 1936 established themselves by swimming into the Lakes. Most were unintentionally transported or released.

“It only takes one idiot to infect a location with an exotic [species],” Cuhel said. “One fisherman with a bait bucket can be worse than river flow.”

Cuhel won’t weigh in on policy or engineering proposals to physically separate the Great Lakes and Mississippi basins, or treat locks with chemicals that could clear out carp and other invasive species before they enter Lake Michigan. But others have called expensive efforts to keep out invaders a foolhardy investment.

There are dozens of species in the Great Lakes basin that don’t currently exist in the Mississippi, and nearly a dozen more vice versa. Aquatic invasive species protections could defend those populations from cross-contamination. What’s more, environmental agencies already spend hundreds of millions of dollars each year managing algal blooms, sea lamprey and mussels. That makes an economic argument for prevention measures, Chadderton said, even if the carp don’t turn out to be good colonizers of most Great Lakes waters.

“The trouble with any invasion is that there will always be evidence on both sides. So do you let the experiment run?” Chadderton said. “The most prudent management option is to prevent establishment.”