The Ocean

Photo: Fish and corals

Unknown in the Americas until three decades ago, lionfish—such as the one photographed here near the wreckage of the U.S.S. Schurz in North Carolina—have since been spotted from Rhode Island to Belize.

Photograph by Michael Madelung, My Shot

Aquatic invasive species are plants and animals that evolved in one location and are introduced through a variety of means into another location.

Species have always used the oceans to move about the planet. By swimming or hitching a ride on a log, leaf, or coconut, organisms have found new worlds in which to thrive. But until recently, this process has been moderate, limited by the currents and the winds.

Since humans first took to the seas, though, intrepid stowaways have had ever expanding vehicles for dispersing themselves both faster and farther. The result is an increasing number of ocean ecosystems, primarily near shorelines, that are being compromised or wiped out by non-native species.

What Are They?

An invasive, or non-native, aquatic species can be any organism that exists somewhere in or near water where it doesn't belong. When an alien species like this arrives in a new location, several things can happen: It can find its new habitat unwelcoming and die off; it can survive with little environmental impact; or it can take over, harming the naturally existing wildlife in a variety of ways.

Invasive species that thrive usually do so because their new habitat lacks natural predators to control their population. They do damage mainly by consuming native species, competing with them for food or space, or introducing disease.

One infamous example is the zebra mussel, accidentally introduced by a cargo ship into the North American Great Lakes from the Black Sea in 1988. The tiny mollusk multiplied uncontrollably, starving out many of the Great Lakes' native mussel populations and interfering with human structures from factory intake pipes to ship rudders. They've now spread from Canada to Mexico and are considered a major nuisance species. Hundreds of millions of dollars are spent annually to control their numbers.

How Do They Get There?

Most marine invasive species stow away in ship ballast. Large boats have tanks in their hulls that are filled with seawater to counterbalance cargo weight. Boats draw in water at their loading port, in some cases more than 20 million gallons (75 million liters). When the ship arrives at its destination, it releases the ballast—along with whatever species happen to be inside, from schools of fish to microscopic organisms. And these days, there's plenty of opportunity for hitchhiking species. Some 45,000 cargo ships move more than 10 billion tons of ballast water around the world each year.

Invasive species also hitch rides on the outside of ship hulls and on the millions of tons of plastics and other trash that floats around the globe in ocean currents.

Pets acquired through the aquarium and exotic pet trade—and then released—can become invasive species, as can escapees from aquaculture farms.

And ongoing sea temperature rise caused by global warming is allowing non-native species to populate ocean habitats that were once too cold to be hospitable.

Combating Invasives

To combat invasive species, governments are focusing on how they handle ship ballast. New regulations in a handful of countries require ships to exchange their ballast water while out at sea or treat it to kill stowaway species before they are released.

Share

Did You Know?

Zebra mussels can congregate in masses as dense as 70,000 individuals per square foot.

More About Invasive Species

  • Photo: Close-up of a hippo

    Cocaine Hippos

    One of the many hippos of an arrested cocaine drug lord that are breeding uncontrollably in Colombia and local wildlife conservationist are struggling to control this un-welcome visitor.

  • Photo: Emperor penguins

    Alien Species Invading Antarctica, Experts Warn

    Scientists raise a call to arms to prevent invaders from transforming Antarctica's unique ecosystems.

  • Photo: Swarm of jellyfish

    Jellyfish Gone Wild

    Certain types of human-caused environmental stresses are triggering jellyfish swarms in locations around the world.

Why We Need Marine Reserves



Ninety percent of the large predators in the ocean are gone and their populations have collapsed. The reason for this is that we have taken too many fish out of the sea, and we keep taking more before the remaining populations are able to reproduce.


Watch this video where Mel, the “very weird” fish, will show you how marine reserves can help fish populations recover, and why we need many more.


Learn More

We are committed to protecting the last wild places in the ocean.

Learn More »

Your Ocean

  • Photo: Clown anemonefish

    For Kids

    Learn about the ocean with activities, photos, and games.

  • Photo: A school of fish and a shark swim in a coral reef.

    Ocean Education

    Bring engaging and important ocean learning to your classroom.

Support the Ocean

Explore the Ocean

  • shark-eden-ocena-store-promo.jpg

    Ocean Life

    Order ocean books, DVDs, maps, and more from the National Geographic online store.

  • Photo: Leopard seals on a glacier

    Ocean Special Issue

    Explore the world's oceans, from their prehistoric beginnings to modern-day efforts to preserve their natural wonder.

  • Photo: Ocean Atlas from National Geographic

    Ocean: An Illustrated Atlas

    Immerse yourself in the wonders of the deep through colorful maps, photos, and satellite images.

Engage, Conserve, Restore

  • Photo: Sunset at waterfalls

    Freshwater Initiative

    The National Geographic Society’s freshwater initiative is a multi-year global effort to inspire and empower individuals and communities to conserve freshwater and preserve the extraordinary diversity of life that rivers, lakes, and wetlands sustain.

Pristine Seas logoPristine Seas: Protecting the Last Wild Places