More than half of San Francisco Bay fish and most of its bottom-dwelling organisms are not native to the Bay, and new invaders are constantly being introduced.
Invasive species are non-native species that have been introduced into a new landscape, freshwater system or ocean region. Because this new area often lacks natural competitors and predators, the invaders tend to displace native plants and animals, disrupt food webs, and alter fundamental natural environmental processes.
Chinese mitten crab, Eriocheir sinensis, taken from San Francisco Bay (Photo courtesy California Dept. Fish and Game)
The Chinese mitten crab is one of more than 200 exotic species which have invaded the Delta and San Francisco Bay since the 1850s. These small crabs, which were brought to the San Francisco Bay via ships' ballast water, were first were documented in California in 1992. They reproduce rapidly and have spread throughout the Delta. They may imperil the state's threatened and endangered salmon populations due to the crabs' appetite for juvenile salmon.
"The scale of this problem is vast," said Jennifer Molnar, conservation scientist at The Nature Conservancy and lead author of the study, "Assessing the Global Threat of Invasive Species to Marine Biodiversity," published in the journal "Frontiers in Ecology and the Environment."
"Every day, thousands of vessels cross our oceans with invasive species hitchhiking on their hulls," Molnar said. "Because of this, as many as 10,000 species are estimated to be in transit at any one time."
The first quantitative global assessment of the impacts and causes of marine invasive species, the study evaluates where they are located, how they are transported, and which species are the most harmful to native habitats.
"Once alien species become established in marine habitats, it can be nearly impossible to remove them," said Molnar. "The best way to address these invaders is to prevent their arrival or introduction in the first place."
Cargo ship entering San Francisco Bay may be carrying invasive species on its hull and in its ballast water. (Photo by J. Greenman)
The study, analyzes the problem on a global scale by synthesizing information on 329 aquatic species, drawing information from over 350 data sources.
The report is accompanied by a geographically referenced and publicly available database of marine invaders that can be used to determine the most threatening species per region, and to prioritize strategies for preventing further damage.
Hawaii is named as an ecoregion with high levels of invasion, with 73 marine invasive species, 42 percent of which are considered harmful and are disrupting multiple species or wider ecosystems.
The study only includes species for which there is scientific data available on the ecological impact, and local scientists believe there may be many more marine invasive species in Hawaii.
Likely pathways for the spread of these invasive species in Hawaii are shipping and aquaculture, according to the study.
"Many in Hawaii have seen what alien algae have done to some of our reefs. We need all the tools that can be provided for wise management of our coasts," said Celia Smith, a professor and seaweed specialist in the University of Hawaii's Botany Department. "Our next concern is that reefs in other parts of the Pacific may be similarly impacted but have no scientists there to help."
This reef in Kaneohe Bay on the island of Oahu is choked by the invasive algae Eucheuma denticulatum. (Photo courtesy Nature Conservancy)
An effective tool in combating invasive algae in Hawaii is the Super Sucker, an underwater vacuum cleaner that sucks invasive algae off the reef and places it on a barge above water so that non-invasive marine life can be sorted and returned to the water.
The alien algae is packed into sacks and delivered to taro farmers for use as fertilizer. The full-size device can scoop up about 800 pounds of algae an hour.
Currently, there are two bills moving through the State Legislature that, if passed, will appropriate funds for the full-time operation of the Super Sucker in Kaneohe Bay for one year and for purchase of a portable unit.
"The Super Sucker is an essential component of a comprehensive management strategy for controlling these alien algae," said Eric Conklin, marine science advisor at The Nature Conservancy in Hawaii. "The research we've done shows that we can efficiently remove mass quantities of algae from impacted reefs. In some cases, we can restore the reef to a condition that native fishes are able to maintain in an algae-free state, which allows corals to recover."
The economic costs of invasive species, including marine invaders, can be enormous. The United States spends $120 billion annually to control and repair damage from the country's more than 800 invaders.
Caulerpa, a tropical seaweed that has invaded the Mediterranean and in Australia, is transported on the anchors of fishing and recreational boats. It is toxic to many fish and spreads rapidly, eliminating native seagrass. When a caulerpa population was discovered in a California harbor in 2000, divers needed six years and over $4 million to eradicate it.
Throughout the world's oceans, aquatic alien invasives damage economies by diminishing fisheries, fouling ships' hulls, and clogging intake pipes. Some can even directly impact human health by causing disease.
The study cites many examples of this damage, such as the comb jellyfish that was carried to the Black Sea on a ship in the early 1990s. By feeding on fish eggs and zooplankton, they devastated Black Sea fish populations and disrupted the entire food chain.
Comb jellyfish (Photo courtesy Nature Conservancy)
At its peak, this plague made up 90 percent of the weight of all living organisms in the Black Sea. When the comb jelly invasion later reached the Caspian Sea, it again diminished fish populations and left the threatened Caspian seal both hungry and vulnerable.
Both of these invasions destroyed commercial fisheries and caused coastal communities to lose thousands of jobs.
Native salmon populations in Scotland and Scandinavia are being infected by new pathogens, while escaped farm salmon are weakening the genetic resilience of the wild salmon, according to the study. Each year, up to half a million salmon escape from fish farms in Norway alone.
The Nature Conservancy is working as part of the Global Invasive Species Programme, a coalition of four international environmental organizations, to stop further introductions of marine invasive species. The Conservancy also advises policymakers on how to develop prevention strategies at ports and on shipping vessels.
The upcoming 9th Conference of the Parties of the Convention on Biological Diversity, scheduled for Bonn, Germany in May, will conduct an in depth review of work to date on invasive alien species.
The study, "Assessing the Global Threat of Invasive Species to Marine Biodiversity," is online here.
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