The zebra mussel has stood as an example of an invasive species since it wreaked economic and ecological havoc on the Great Lakes in the late 1980s. Yet despite intensive efforts to neutralize it and its relative, the quagga mussel, this fingernail-sized mollusc spreads in rivers, lakes and bays in the United States and Canada, especially in the St. Lawrence River, clogging water supply pipes and altering food webs.
Today, mussels threaten to reach the last major uninfested freshwater areas in the western and northern United States: in Washington and Oregon, the Columbia River Basin, and the waterways of Alaska.
As an environmental historian, my job is to study how people’s attitudes towards non-native species have changed over time. Like many other aquatic intruders, zebra mussels and quagga mussels spread into new bodies of water, carried by people, either accidentally or deliberately. Human-made structures, such as canals, and detrital material can also help invaders circumvent natural barriers.
In my view, to reduce the damage caused by these epidemics — and prevent them wherever possible — one must understand that human activities are the root cause of costly biological invasions.
Transoceanic invasions of the past
Exploration of the Americas by Europeans between the late 1400s and 1700s resulted in massive transfers of organisms, a process known as the “Columbian exchange”, named after Christopher Columbus. Many investors got rich by shipping cattle and plantation crops across the oceans. These transatlantic voyages also brought in germs that caused infectious diseases, such as smallpox and measles, which killed millions of Aboriginal people who were not immune.
During the 19the century, European and North American colonizers established acclimatization societies to import desired species of foreign plants and animals to use for food, sport hunting, or to beautify their surroundings. Many of these efforts have failed. As introduced species failed to adapt to their new conditions, they did not survive.
Other events have been at the origin of legendary ecological disasters. For example, after the Victorian Acclimatization Society released European rabbits into Australia in 1859, they multiplied rapidly. Feral rabbits, along with other introduced species like cats have destroyed millions of Australia’s native plants and animals.
Shipping has also contributed to the accidental spread of alien species. Man-made canals facilitated the transport of goods, but also provided new access routes for aquatic parasites.
At the end of the 19e and at the beginning of the 20e, for example, Canada expanded the Welland Canal between Lake Ontario and Lake Erie to allow large ships to bypass Niagara Falls. As early as 1921, these technological improvements enabled the sea lamprey, a parasitic fish, to move from Lake Ontario to the upper Great Lakes, where it still poses a serious threat to commercial fishing operations.
In 1959, the United States and Canada opened the St. Lawrence Seaway, a navigation network that connects the Atlantic to the Great Lakes. Ocean-going ships that use the Seaway have carried stowaway species in their ballast water — tanks full of water used to keep ships stable at sea.
When ships reach their destination and empty their ballast tanks, they release alien plants, crustaceans, worms, bacteria and other organisms into local waters. In a landmark 1985 study, Williams College biologist Jim Carlton described how ballast water discharges were a powerful vehicle for biological invasions.
The invasion of the Great Lakes by mussels
The zebra mussel is native to the Black Sea and the Caspian Sea. It is thought to have entered North America in the early 1980s and was officially recorded in the Great Lakes in 1988, followed by the Quagga mussel in 1989.
Quickly, the striped bivalves covered the hard surfaces of the lakes and washed up on the shores, inflicting cuts on the feet of swimmers. Zebra mussels have clogged intake pipes at drinking water treatment plants, power plants, fire hydrants and nuclear reactors, dangerously reducing water pressure and requiring costly repairs.
Molluscs are filter-feeding organisms that generally make water clearer. But zebra and quagga mussels filter so much plankton out of the water that they starve native mussels and promote harmful algal blooms. Invaders also transmitted type E botulism that is deadly to fish-eating birds.
By the early 1990s, 139 alien species had established themselves in the Great Lakes, nearly a third of them after the opening of the St. Lawrence Seaway. Ship-related introductions, as well as other pathways, such as aquaculture and the release of aquarium and baitfish, have transformed the Great Lakes into one of the most heavily invaded freshwater ecosystems in the world.
First political reactions
The United States began regulating ballast water management in 1990, but has struggled to close the loopholes. For example, ships declaring that they had no pumpable ballast water on board did not have to empty and refill their tanks with clean seawater during the voyage. As a result, living freshwater organisms hiding in reservoir sediments could still be released into vulnerable ports.
Finally, after extensive studies, the United States and Canada in 2006 required ships to flush tanks containing residual sediment with seawater. A 2019 assessment found that only three new species became established in the Great Lakes between 2006 and 2018, and that none of it was through ships’ ballast water.
Today, other anthropogenic practices are increasingly contributing to harmful introductions into freshwater. And with marine activities being regulated, the big culprits are thousands of private boaters and anglers.
Stemming the Westward Spread
Zebra and Quagga mussels move west and south of the Great Lakes, tethered to private boats or carried in bilge waters and bait buckets. They have been discovered in Nevada, Arizona, California, Utah, Colorado and Montana.
If mussels reach the Columbia River ecosystem, they will be a threat to native wildlife and to irrigation pipes and dams vital to agriculture and hydroelectricity. Government officials, wildlife managers and scientists are working tirelessly to prevent such a situation from happening.
Public awareness is essential. Travelers who transport their boats without decontaminating them can transfer zebra and quagga mussels to inland river and lake systems. Mussels can survive out of water in warm places for weeks. It is therefore important that boaters and anglers clean, drain and dry their boating equipment and fishing gear.
Aquarium owners can help stem the tide by disinfecting their tanks and accessories to prevent accidental releases of live organisms into public waterways, and by being vigilant when purchasing. In 2021, zebra mussels were detected in imported moss balls sold as aquarium plants in the United States and Canada.
Maintain public support
Some of these efforts have yielded encouraging results. Since 2008, Colorado has had a rigorous boat inspection program in place that has kept zebra and quagga mussels out of state waters.
But prevention is not always welcomed. Authorities closed the San Justo Reservoir in central California to the public in 2008 after discovering zebra mussels there; residents say the closure has harmed the community and are lobbying the federal government to eradicate the mussels in order to reopen the body of water to fishing.
Mitigating the destructive effects of invasive species is a complex mission that does not necessarily have an obvious end. It requires scientific, technological and historical knowledge, political will and skills to convince the public that everyone is part of the solution.