The Arms Race Against Invasives

By Michael Vatalaro
Published: April/May 2012

Invasive species often become a nuisance to boaters because they proliferate greatly in absence of natural checks and balances. Now researchers are fighting back with solutions found in nature.

Photo of Water hyacinth, an invasive plant from the Amazon Water hyacinth, an invasive plant from the Amazon, chokes off access to transient slips in Stockton, California.

Zebra mussels, and their close cousins quagga mussels, are exceptional at what they do for a living: filter phytoplankton from the water to feed on. This simple act, which happens at the rate of around a quart of water a day per mussel, can dramatically improve water clarity. But at the same time, these invaders, which hitched a ride in the belly of an ocean-going ship from Europe, are also reducing the food supply for the fish and native mussels that ordinarily rely on it. The small bait fish that feed on phytoplankton in turn are food for predatory salmon, walleye, and smallmouth bass, to name a few, and when the bait population declines, their numbers go down as well.

In the meantime, with a single female zebra mussel capable of releasing one million eggs a year, the little mollusk is taking over. Since being discovered in the Great Lakes in 1988, these pests have made their way into freshwater lakes and river systems in dozens of states, forcing power plants, canal and lock operators, and municipalities to spend billions of dollars in control costs when the rapidly reproducing mussels clog up water intakes or machinery. Happy to adhere to any hard surface, such as hulls, running gear, or culverts, mussels must be physically scraped off, or poisoned with chlorine in the case of water intakes, which is also toxic to most other forms of life in the water. The Great Lakes states alone spend $100 to $400 million each year on control efforts. An estimate of costs associated with setting up chemical control systems at one large municipal water facility ranged from $2-4 million in initial costs with $500,000 to $850,000 per year to be spent on chemicals.

But now, researchers have come up with a way to use the mussel's extraordinary filtering abilities against it, which for the first time could provide a means for large-scale eradication without collateral damage, and cost reductions.

The Silver Bullet

For more than a decade, researcher Dan Molloy at the New York State Museum Cambridge Field Lab has been investigating a natural toxin that is deadly to zebra and quagga mussels, but doesn't harm fish, vegetation, or other wildlife. A strain of the bacteria Pseudomonas fluorescens, when ingested by a mussel through filter feeding, kills it within a week. The bacteria doesn't even have to be alive — it doesn't infect the mussel with a disease. The dead bacterial cells themselves are the toxin. What's more, this toxin is incredibly specific to zebra and quagga mussels. Native mussels aren't harmed when exposed.

After two years of field tests in Colorado and Ontario, where treatments averaged 70 percent mortality rates, the Environmental Protection Agency (EPA) granted approval to a commercialized version of the toxin called Zequanox in September 2011. The approval opens the door to more widespread use of Zequanox.

Closeup photo of a water hyacinth Water hyacinth

Unlike chemical treatments, typically using chlorine, Zequanox won't harm the environment, or persist in the form of carcinogenic byproducts. Adding chlorine to a water body is basically the nuclear option; it will kill just about everything that swims, grows, or floats nearby. But Zequanox's selective toxicity means that authorities could at least consider treating smaller water bodies in their entirety, creating the possibility of eradication of zebra or quagga mussels from an infested lake. Incidentally, there is only one recorded instance of complete eradication of zebra mussels from a lake in the U.S., and it required 174,000 gallons of potassium chloride to treat a 12-acre quarry lake at a cost of more than $400,000, which also killed all other species of mollusks in the quarry.

Mussels aren't the only species in the "bio-control" crosshairs. The U.S. Department of Agriculture recently approved an experiment in California's Sacramento River Delta to combat an invasive plant called water hyacinth.

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