The Arms Race Against Invasives

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.

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 "biocontrol" 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.

From Pretty Flower To Choking Weed

Water hyacinth was brought to the U.S. for the 1884 Cotton States Exposition in New Orleans, as a beautiful curiosity, from its native Amazon waters in South America. Exhibitors reportedly passed out samples of the plant, after which it was carried across the southeast by the visiting audience. By 1895 water hyacinth had formed several 25-mile long mats in the St. Johns River in Florida, blocking navigation on portions of the river.

This invader has established itself across Florida and the southeast, in the Sacramento River Delta in California, as well as in Washington. One of the fastest growing plants known, water hyacinth can double its population in two weeks. In nutrient-rich waters, it will form thick mats that block sunlight from reaching the bottom. This in turn can completely inhibit photosynthesis, killing native vegetation and phytoplankton alike, decimating the food chain. The plankton die-off will cause the oxygen levels in the water to drop, which can cause fish kills. Researchers have documented harmful effects with as little as 25 percent surface coverage of water hyacinth. Left unchecked, water hyacinth will completely close off a water body to boat traffic or swimming.

The Hungry Army

The U.S. Department of Agriculture recently approved an experimental, biological treatment for water hyacinth; called the water hyacinth plant hopper, it's a tiny insect native to the same Amazon waters. The state of California petitioned the USDA for approval to use the plant hopper against water hyacinth in the Sacramento River Delta. Plant hoppers appear to be ideal as a biological control because they only feed on water hyacinth. After extensive testing in quarantine, the USDA approved the petition, and the first 6,000 or so insects were released beginning in May of 2011. The project is being managed by the California Department of Food and Agriculture (CDFA), which reports that the plant hoppers are reproducing as expected in the wild, adding about one additional generation per month.

"Water hyacinth is a serious problem not just for agriculture and our state's water supply, but for anyone who appreciates the natural beauty and recreational value of our waterways," CDFA Secretary Karen Ross said when announcing the project. "CDFA's new biocontrol project will help reduce the impacts of this invasive weed in the Delta."

The hot summer didn't appear to set the plant hoppers back, but the winter months could. Plant hoppers feed on water hyacinth by sucking the plant's juices from its leaves, much like aphids destroy rosebushes. If enough plant hoppers feed on the same leaf, it wilts and dies. The CDFA hopes that the plant hopper colonies will prove hardy enough to survive on their own. If they prove viable, a larger-scale project to reduce water hyacinth would be launched.

A Cautionary Tale

Biological agents that are pest-specific such as the water hyacinth plant hopper and the toxic bacteria in Zequanox are the holy grail in invasive-species management. When less specific natural agents are introduced, things can go awry. In 2005, Lake Conroe in Texas was again facing an invasion of hydrilla, another nusiance water plant that grows in mats that blanket the surface, preventing sunlight (and boats) from passing. Hydrilla covered more than 9,000 acres of Lake Conroe in the late 70s, at one point completely encircling the shoreline, leading the Lake Conroe Association to fight back. Expensive herbicide treatments seemingly did little, so the association hit upon the plan of introducing the white Amur, a grass carp species, into the lake to eat the hydrilla. The fish ate the hydrilla and later died off. It worked well enough that they reenacted the plan again just a few years ago. After raising more than $500,000 to purchase and stock 100,000 triploid white Amur (which can't reproduce) things seemed to be on track. The voracious plant-eating fish reduced the hydrilla population by 99 percent, leaving just two acres of the weed after only two years.

The problem is, after exhausting the readily available hydrilla supply, the carp kept eating. The hungry fish also ate 80 percent of the beneficial native water vegetation. Now the lake's managers are faced with finding the balance between the number of white Amur, and the amount of hydrilla, which sprouts annually. In the 1970s, the ratio of fish stocked to acres of hydrilla was about 30 to 1. In the last stocking effort, it was about 50 to 1. This past summer, the Texas Parks and Wildlife Department held a grass-carp-removal fishing tournament to target the otherwise protected species. With an estimated 32,000 or so white Amur remaining, the goal is to balance the number of carp with the several hundred acres of hydrilla that emerge each year.

Doing Our Part

While biological controls hold promise, it's much more effective to prevent the spread of invasive species the first place. Stopping the spread of zebra mussels, hydrilla, or water hyacinth isn't just good boating practice, it's becoming mandatory across the country. Regardless of how they got here, they're our problem now. Recreational boaters stand to lose out no matter where we live, if these species go unchecked.

For boaters, invasive plants such as hydrilla, water hyacinth, and milfoil can blanket the surface of our waterways, forming an impenetrable, prop-snarling mess. When Asian carp leap from the water like popcorn from a hot skillet, it's fishermen, boaters, and waterskiers who are endangered. Mussels can clog water intakes and cooling passages, wrecking inboard engines. By law we're required to do what we can to stop these critters in their tracks and this, for boaters, has to happen at the boat ramps across the country.

Inspector Veliger At Your Service

Boat inspections, whether courtesy or mandatory, are common at ramps around the country, particularly in western states. States as varied as Maine and Wyoming have instituted aquatic-nuisance species (ANS) decal requirements for boats to generate funds to pay for inspection programs, while others pay for inspections through general boating funds.

A typical boat inspection starts with a short list of questions to determine whether your boat presents a low or high risk of transferring an unwanted pest. These questions include which body of water your boat was last used in, how long it's been out of the water, and whether or not you took steps to "Drain, Clean, and Dry" your boat as recommended. If your answers indicate your boat is a low-risk boat, a quick visual inspection for vegetation hanging from your trailer might be the only additional step, getting you on your way to the ramp within a few minutes. If your boat is determined to be high-risk, you might be pulled from the ramp line for a more thorough, hands-on inspection that could result in a decontaminating, hot-water wash down.

The Next Wave

The need for inspections and good boat-cleaning practices continues to grow as the next wave of invaders comes ashore. The New Zealand Mudsnail was found in western rivers, including the Snake River in Idaho in the late-80s. It has since spread to several other river systems, including the Madison River near Yellowstone National Park. These small snails, typically only a few millimeters long, can reach densities of 750,000 per square meter, and have the potential to disrupt the food chains in some of our nation's best trout-fishing rivers. Anglers who wade in these streams also need to take precautions to clean and dry their tackle and waders between uses to prevent the spread of the snail.

Didymo (or "rock snot") is a more recent invader, found in western rivers in 2004 and in Tennessee in 2005. It has since spread to at least 18 states. This freshwater algae can bloom in numbers sufficient to block out sunlight and coat surfaces. It's easily spread by contaminated fishing gear as well as by boats. Hot water (140+ degrees) decontamination will kill didymo on boats. Waders and other fishing tackle should be soaked in a five percent solution of water and dishwashing detergent (2 cups detergent to 2.5 gallons water) for 30-40 minutes. Or tackle should be allowed to dry for at least two days.

While states are spending more time and effort to inspect boats at ramps, they're also investing in mobile and fixed decontamination units. Minnesota rolled out three mobile decontamination trailers last August as part of a pilot project to compliment their ramp inspection program, which has been in place since the 90s. The $15,000 trailers hold, heat, filter, and recycle 400 gallons of water, which is kept at around 160 degrees, and can spray this scalding water at 3,000 psi. The whole thing is powered by a 4-kw generator and needs to be operated by a trained professional. The trailer and the large black pad that the boat trailer needs to be parked on for water capture and recycling attract a lot of notice at the ramps. The trailers rotated through about 30 state-owned ramps last season.

"We answered a lot of questions and cleared up some misconceptions about the program," says Heidi Wolf, the watercraft inspection coordinator with the state DNR. "Everyone was positive and supportive. Boaters expect to get inspected. Now we can decontaminate their boats, rather than turn them away." The Minnesota DNR expects to purchase 20 additional units for use this boating season.

The more we do as boaters to slow or stop the spread of aquatic invasive species, the more time there is to discover innovative solutions such as biological controls. With the help of comprehensive inspection and cleaning programs like Minnesota's, boaters can help turn the tide against aquatic invaders.