Ship hulls a major source of invasive species introduction in California waters
Evolution and Ecology Professor Susan Williams and colleagues have conducted the first-ever multi-source study of invasive marine species origins in Northern California, identifying a previously underestimated threat to the state's native marine species: Ship hulls.
Out this month in the journal BioScience, the article "Managing Multiple Vectors for Marine Invasions in an Increasingly Connected World" is the first analysis comparing the different ways that invasive species arrive in new waters. The results will inform the development of new ways to protect California's bays and estuaries from invasive species.
Williams and her colleagues' analysis indicates that invasive species arriving on the outside of ship's hulls, called "biofouling," account for more species introduced into Northern California than the next two largest sources combined. And no matter how the team calculated the contributions of the seven major sources studied, called vectors, biofouling was attributed with about half of all introductions.
When docked, hulls experience colonization by animals and plants that grow on their outer surface— "the stuff that boat owners try to scrape off," Williams explained. These species are introduced into new waters as the ships travel from port to port. If the hull life colonizes the new waters, it can disrupt the local ecosystem and cause great harm to the native populations there.
"Previously, the focus has been on ballast water," Williams said. "There are regulations and guidelines in place for controlling the spread of species from ballast water. And people do know that invasive species can arrive on the hulls of ships, but there are currently no management strategies for that."
The best way to prevent invasive species from causing harm is to pinch off the vectors delivering them. Most management tackles a single vector at a time, with ballast water being the primary focus, but in reality many vectors operate simultaneously.
Williams and her colleagues' study was the first comparison of the relative risk posed by multiple vectors such ballast water, vessel hulls, aquaculture and the aquarium trade, for marine invasive species. Further research will attempt to quantify the exact threat each vector poses, but in the meantime their results are a call to take immediate action to curb biofouling.
"Right now we really need to shift that management focus onto biofouling and hulls, which have been ignored," Williams said. "It is still necessary to manage ballast water, but there is a great need to begin addressing biofouling too."
Luckily, that interest dovetails with economic interests of boat owners.
"It is very costly to have a dirty hull—one estimate is that if your hull is not clean, your fuel costs increase 40 percent each voyage," Williams said. "And as the oceans have warmed, the species are growing bigger and faster, so there is a strong economic incentive to control hull species growth."
She added that the State of California has taken note of her team's results and is discussing plans to increase education about biofouling and hull maintenance.
The issue is especially important for California's Bay Area, where "it is hard to find a native species," according to Williams.
"Bays are the hot spots for invasive species worldwide," she added, mentioning the case of the Asian clam, which was introduced into San Francisco Bay—probably via ballast water—about three decades ago. After its arrival, the clam filtered all the phytoplankton from the bay waters, drastically changing the structure of the food web and the species of fish that live there.
In November, Williams received the Distinguished Service Award from the Coastal & Estuarine Research Federation, an international scientific society dedicated to research and education about coastal marine science and management.
The award recognized her long career of working to maintain the health of marine systems. She served a six-year term at the head of the organization, during which time she also organized two Capitol Hill briefings, one on the Deepwater Horizon oil spill, and the second on public health linkages to coastal environments.
Williams' research was supported by Proposition 84 funds made available to the California Ocean Science Trust by the California Ocean Protection Council, with additional funding from the California Ocean Science Trust, the California Sea Grant Program, and the Smithsonian Institution.