Aquatic invasive species (AIS) are an economic and environmental danger that threaten all water bodies, and the St. Croix River is no exception.

Invasives are defined by federal law as species that are non-native to an area that are likely to cause harm to the economy, environment, or human health. The St. Croix River already hosts populations of species such as Asian carp, banded mystery snails, and curlyleaf pondweed, each of which was likely introduced by humans.

When introduced to a foreign ecosystem, invasive species tend to flourish due to a lack of natural predators or disease controls, and often crowd out native species as they take over food sources and space.

Very often, once established, there is very little to be done to eradicate them.

“The problem is that these invasive species are so good at adapting to these environments that…we often don’t have the people or resources to stop them,” said Trevor Cyphers, a fisheries biologist with the U.S. Army Corps of Engineers.

Education and prevention, therefore, are often just as important, if not more so, than restoration of already-infested ecosystems.

One particular invader that gets a lot of attention is the zebra mussel (Dreissena polymorpha). This small, freshwater mussel typically grows only two inches long at most, yet can become a massive problem in lakes and slow-moving rivers due to its ability to create a lot of offspring very quickly.

Byron Karns, Acting Chief of Resource Management for the St. Croix National Scenic Riverway, a unit of the National Park System, said that the ecological damage of a zebra mussel infestation fits many of the common patterns of an invasive species. Once in an area, their populations explode, and they quickly begin to overwhelm niches normally occupied by native species of mussel.

The zebra mussels will outcompete other mussels for food and space, and sometimes smother the natives by latching onto them and preventing them from moving, feeding or reproducing properly.

Zebra mussels can also cause economic damage.

A good example is Bass Lake in St. Croix County, Wisconsin. Bill Holmberg is a resident living on the edge of the lake, which was diagnosed as infested with zebra mussels in 2010. Ever since the invasion, the local residents have had to deal with the economic consequences.

Biofouling, or the accumulation of zebra mussels on surfaces put in the water, is a notable example.

“The biggest issue for owners like myself is they cover everything you put in the water,” Holmberg said.

Boats are less of a problem because they’re usually removed from the water before the microscopic mussel larvae, or veligers, have a chance to latch on. Docks, however, are oftentimes heavily coated since they remain in the water for the entire season, and any water that gets trapped within the motor of a boat when it’s tipped back might also contain larvae waiting to anchor themselves and clog up the system.

Other, less obvious problems have arisen from the infestation at Bass Lake. Residents with sprinkler systems that draw from the lake have to invest in special filters to keep the mussels out. Such a filter might cost nearly $100, but to go without is to risk losing the entire sprinkler system and spending $5,000 on a new one.

The sharp, dead shells can also be a safety hazard to anyone walking barefoot on a beach, and the pump system normally used to keep the water level of the lake from overtaking nearby properties cannot be turned on for fear of spreading the mussels to other water systems.

Zebra mussels have already made their way into the St. Croix River, but so far they have not progressed north of Stillwater. Nor can they, without help from humans.

Karns said that due to the method by which zebra mussels reproduce (spewing veligers into the water and allowing them to drift with the currents), they typically can’t move upstream on their own. To move up a river or from lake to lake, zebra mussels hitch rides with boaters or fishermen transporting water. Livewells, bait buckets and boats filled with water from an infested water body might contain microscopic veligers, which have the potential to become an entirely new infestation if released into an otherwise clean environment.

Prevention, Karns said, is the most effective way to address this problem.

A zebra mussel infestation, once it takes root, cannot be eradicated from an entire lake or river. Manual removal is impossible due to the veligers. Chemical treatments like copper sulfate and Zequanox can be effective in small, contained systems, but run into problems like cost and the risk of harming native wildlife when applied to an entire lake or section of river.

“There’s no such thing as a whole lake treatment,” Karns said.

Multiple natural resource organizations have mobilized efforts to prevent further spread of zebra mussels and other aquatic invasive species.

Wisconsin’s Clean Boats, Clean Waters program is one such effort operating within the St. Croix River watershed. The program organizes groups of volunteers and employees to serve as watercraft inspectors, and posts them at boat launches to remind people to inspect their equipment for plants and animals and to not transport water or live bait.

Controlling aquatic invasive species as a whole, Karns said, is going to require a combination of methods. Restoration of already-infested areas is an important aspect of the process, but oftentimes more important is education and prevention.

“I think in some respects it’s something you have to have buy-in from the public,” he said. “Education is critical, in the beginning especially.”

The Bass Lake Rehabilitation District, in response to its infestation, has been implementing methods to educate people and prevent the further spread of zebra mussels. The 2016 Bass Lake Management Plan included a section on AIS, and a DNR grant has allowed them to purchase a camera for their boat landing that not only monitors for boats with AIS, but also serves as a reminder for people to inspect their equipment.

“Be diligent in the prevention,” Holmberg said. “If there’s a public access…don’t just educate the users, try to educate the public in general.”

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