First Ever Global Maps Reveal Spread and Severity of Marine Invasive Species

Eighty-four percent of the world’s coasts are being affected by foreign aquatic species, according to a new Nature Conservancy study published today in the journal Frontiers in Ecology and the Environment.

This study represents the first quantitative global assessment of the impacts and causes of marine ‘invasive species,’ and evaluates where they are located, how they are transported, and which species are the most harmful to native habitats.

“Everyone in the world depends on healthy oceans and coasts for survival. Invasive species are severely impacting native plants and animals, and are causing significant economic damage at the same time,” said Stephanie Meeks, acting president and CEO of The Nature Conservancy. “By understanding the scale and scope of these invaders, we are better equipped to stop them.”

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 invasive species tend to displace native plants and animals, disrupt food webs, and alter fundamental natural environmental processes.

The economic costs of invasive species, including marine invaders, can also be huge. The U.S. spends $120 billion annually to control and repair damage from the country’s more than 800 invaders. 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 voraciously on fish eggs and zooplankton, they devastated fish populations and disrupted the entire food chain. At its peak, this plague made up 90% 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.

“The scale of this problem is vast,” said Jennifer Molnar, conservation scientist at The Nature Conservancy and lead author of the study. “Every day, thousands of vessels cross our oceans with invasive species hitchhiking on their hulls. Because of this, as many as ten thousand species are estimated to be in transit at any one time.”

According to the study, international shipping and aquaculture represent the major means of harmful species introduction world-wide, and 80% of all invasives introductions are accidental. Other specific findings from the study include:

- Prized for their plump meat, Pacific oysters have been transported from Japan to be farmed in coastal waters around the world since the early 1900s. Once introduced, they aggressively attach themselves to rocks and group together, squeezing out other species. In Australia and elsewhere, this fast-growing species can smother prized native oysters and mussels, hurting local fisheries.

- Caulerpa, a tropical seaweed that has wreaked havoc in 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 population was discovered in a California harbor in 2000, divers needed six years and over $4 million to eradicate the species.

- Native salmon stocks in Scotland and Scandinavia are being decimated by new pathogens, while escaped farm salmon are weakening the genetic resilience of wild salmon. Each year, up to half a million salmon escape from fish farms in Norway alone.

- San Francisco Bay, California, is the most invaded aquatic region on earth. More than half of its fish and most of its bottom-dwelling organisms are not native to the Bay, and new species are being introduced at an alarming rate.

The study, “Assessing the Global Threat of Invasive Species to Marine Biodiversity,” analyzes the problem on a global scale by synthesizing information on 329 aquatic species and drawing information from over 350 data sources. The report is also 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.

“Once alien species become established in marine habitats, it can be nearly impossible to remove them,” added Molnar. “The best way to address these invaders is to prevent their arrival or introduction in the first place.”

The Nature Conservancy is working as part of the Global Invasive Species Programme, a coalition of four leading international environmental organizations, to stop further introductions of marine invasive species through pathways such as the discharge of ballast water, which ships carry for stability, and the bio-fouling of ships hulls, which occurs when aquatic species hitchhike to new places on the bottom of ships. The Conservancy is working around the globe to advise policymakers on how to develop prevention strategies at ports and on shipping vessels.

More information on the report can be found at