North America’s Great Lakes have long been a poster child for the damage caused by aquatic invasive species, and the difficulty of stopping them.
In the chain of lakes that together form the world’s largest freshwater ecosystem, zebra mussels from Russia and Ukraine choke intake pipes for drinking water systems, swarms of spiny waterfleas from Europe and Asia devour native zooplankton, and voracious ruffe (a kind of fish) feast on the eggs and young of native fish.
The number of invasive species in the lakes has climbed inexorably for decades, to roughly 190 species today. Efforts to halt new arrivals seemed to make little difference. Until now.
Over the last 15 years, the number of new invasive species showing up in the lakes has plunged. New research points to one strategy for much of the success: Quite literally, giving invasive species the flush.
A major source of problem species in the Great Lakes has been stowaways on oceangoing freighters arriving from overseas ports. If a ship is traveling without a heavy load, sailors will suck water into holding tanks to give the ship added heft, making it easier to handle in rough seas. But along with the ballast water comes aquatic hitchhikers ranging in size from bacteria to small fish.
If the ship originates in a freshwater ecosystem, it can ferry the creatures unscathed over the inhospitable saltwater of the ocean, before emptying its tanks when it arrives at a port in the Great Lakes. The zebra mussel, water flea and ruffe are all thought to have arrived in this way.
Then, between 2006 and 2008, first Canada and later the U.S., required ships to pause in the ocean, empty their ballast tanks and then flush them with saltwater before entering the waterways feeding into the Great Lakes. Government inspectors checked ship records or the tanks themselves to ensure compliance.
But it wasn’t known whether the strategy was a success, or the latest in a long line of flops. So a pair of Canadian scientists who have long studied invasive species in the lakes took a closer look. They compiled lists of newly-detected invasive species during three different 13-year time periods: 1981-1993, when ballast water was unregulated; 1994-2006 when regulations had large loopholes; and 2007-2019, the era of strict regulation.
The results were striking. During the first two time periods, an average of 22 new invaders were detected (19 in the first period and 26 in the second). After the ballast flushing became mandatory, the number fell to 4 new species in 13 years, a decline of 85%, according to research published earlier this year in the journal Conservation Letters.
While public awareness of invasive species might have helped reduce the effects from people dumping the contents of aquariums or moving contaminated recreational boats, the drop was most evident for creatures associated with ballast water – from 16 and 15 in previous eras to 2 new species more recently.
“I’m aware of no other documented case in which the invasion rate of a large aquatic system has been suppressed through a management intervention,”said Anthony Ricciardi, a biologist at McGill University in Montreal, and the study’s lead author. “This layer of protection is clearly beneficial.”
To make sure the results weren’t skewed by changes in shipping behavior or environmental conditions, the scientists compared ship traffic and lake surface temperatures during the three different periods. They found no significant change that would explain the sudden drop in new species.
Still, the flushing isn’t a perfect solution. The scientists identified at least two recent invaders that likely came from ballast water, a small shrimp-like crustacean and a microscopic animal known as a rotifer. A stronger crackdown could require a combination of flushing and treatment of ballast water by filtering and disinfection. Past studies have found those two strategies together were most effective for dealing with freshwater invaders, such as those taking over the Great Lakes.
Ricciardi, et. al. “Vector control reduces the rate of species invasion in the world’s largest freshwater ecosystem.” Conservation Letters, Jan. 20. 2022.