Salt marsh cordgrass (Spartina alterniflora), used as packing material for Atlantic oysters (Crassostrea virginica), has been introduced into Oregon. The cordgrass continues to spread along the Oregon coast, taking over mudflats and disrupting bird migrations. Shipments of the European oyster (Ostrea edulis) were brought from Washington to France to supplement a low native stock. The virus Bonamia ostrea was thought to have accompanied these shipments and ended up destroying the remaining native stock of the European oyster in France. Although the Chinese mitten crab (Eriocheir sinensis) was banned for importation and aquaculture in the U.S. in the late 1980's, the crab was discovered in San Francisco Bay in 1994. The crab's high market value and resistance to the ban by California aquaculturists suggest that the crab may have been intentionally introduced.

How Do They Get Here?

Most aquatic invasions have resulted from unintentional introductions with boat traffic being the usual mode of transport. In particular, ballast water has been the source of numerous introductions. When ships head out from port, they often fill up their empty cargo tanks with water. This water helps to stabilize the boat as it travel across the ocean and is referred to as "ballast." When the ships arrive at their new destinations, they empty the ballast water in order to make their way into the shallow bays and rivers of the new ports. As you can imagine, this is like transporting a HUGE aquarium across the ocean. Every time a ship releases its ballast water, it also releases tons of little critters and plants into a new environment.

Small boats can also transport around lots of exotic species. In fact, this is how most aquatic invasive species spread from lake to lake in inland environments. An aquatic weed can become tangled around a boat's propeller and then when that boat is launched into another lake, it also brings with it that hitchhiking weed.

Tanker discharging ballast water into harbor. Photo by David Smith

Though boats are the most likely transport mechanism for aquatic invasive species, there are other pathways. Each year, we move around millions of live aquatic plants and animals in the form of aquaculture products, seafood, bait, and aquatic pets. Sometimes researchers will even intentionally introduce an exotic species into the local environment for the sake of fishery stock enhancement, aquaculture experimentation, or habitat restoration. Though these projects are generally done with good intentions, they can often have negative results in the long term (see side bar).

Just how big of a risk to these other pathways pose to our environment? In 1999, researchers from Northeastern University, MIT Sea Grant, and Williams College-Mystic Seaport joined up to conduct a major survey of companies and organizations that import live marine plants and animals into New England. They found that many of these companies (including seafood companies, bait shops, pet stores, marine research stations, public aquariums, and aquaculture facilities) are at risk of introducing invasive species into our local waters. The team later worked with the Massachusetts Bays Program, Smith College, the NEANS Panel, and representatives from the live marine species trade to find ways to prevent future introductions from these types of pathways.

Adapted from "Biological Invaders: Threats from Aquatic Invasive Species and What You Can Do to Keep Them Out of Our Waters," written for Coastlines by Jay Baker.

DETERMINE YOUR RISK!
If you work for a company that imports live marine species, check back soon for an online survey to find out if you are at risk of introducing exotic species into the local environment.