Aquatic Invaders Threaten Biological Diversity and Heritage

By Diane Noserale

From its native habitat in the coastal rivers of Brazil, Salvinia molesta has established itself around the globe in 70 years. In the United States, it was first identified in 1995 in South Carolina. Since then, it has spread to North Carolina and across the southern tier of States to California. This invasive forms dense mats on the surface of tranquil fresh waters, crowding out native species and blocking oxygen and light to lower parts of the water column. When it decomposes, it falls to the bottom of its adopted home, further depleting oxygen needed to support other species. It can withstand a frost but not frozen surface waters.

Often, they are quietly introduced into the environment by the unwitting -- owners when their pet outgrows an aquarium, ships dumping water from their ballast after crossing an ocean, gardeners looking for exotic ornamentals, or travelers trying to hold onto culinary memories. Sometimes, their arrival is planned in an attempt to control other species, native or otherwise, that someone has deemed a problem. Then they take hold -- snakeheads, sea lamprey, zebra mussels, sea squirts, nutria, green mussels, giant salvinia, and Asian carp are just a few non-native aquatic species that have invaded America's lakes, rivers, wetlands, and nearby marine and coastal areas. Lately, it seems that hardly a week passes before scientists identify yet another species in a place it has never before been seen or doesn't belong. Quickly it becomes part of the local lexicon, sometimes with nicknames like "frankenfish."

The number of introduced species has increased more than sixfold from the 1850-1900 period to 1951-96.

How do invasive species manage to adapt, then thrive, and finally take over an environment that is foreign to them? Tolerance to environmental variations, such as differences in temperature, salinity, or chemistry, helps. Perhaps more important, an effective invader often is able to use a wide range of food sources. In other words, picky eaters need not apply. A lack of predators in the new environment or effective ways of fending them off is an important characteristic of successful invasive species.

"The prevalence of invasive species is an unfortunate consequence of our increasingly interconnected world," said Bill Gregg, USGS invasive species program coordinator. "Education, legislation, and enforcement are important and perhaps the most cost-effective tools in preventing invasive species. Where prevention fails, early detection and rapid assessment are necessary to support effective and timely responses. That is where USGS research and expertise comes into the picture," said Gregg.

Northern snakehead gained nationwide fame in the summer of 2002 as "frankenfish" for its ability to wipe out a wide variety of aquatic species. Importation of live snakeheads to the United States, where they were primarily sold in Asian markets, is now prohibited.

In the summer of 2002, USGS biologists worked with colleagues in Maryland to keep the notorious northern snakehead from migrating out of a Crofton pond and into the Patuxent River. The prospect of an invasion of northern snakehead into the Nation's waterways was of great concern, because this species will eat nearly anything and scientists feared there would be no way to eradicate it.

The Wisconsin Department of Natural Resources recently called upon USGS expertise in identifying a non-native fish found in the Rock River. Within minutes of sending a digital image of the strange fish, the Wisconsin DNR got its answer. USGS scientist Walter Courtenay, a Wisconsin native working in Gainesville, FL, identified the fish and replied. The fish was a giant snakehead, a relative of the species found in Crofton. The State called on Courtenay again for advice on drafting regulations to effectively combat dumping of non-native species into State waters.

Despite the cute name, this invader poses a serious threat to the highly productive fishery of Georges Bank off Cape Cod. These siphon-feeding animals form dense mats, made of many thousands of individuals, encrusting and smothering hard sea bottom and organisms attached to it.

This fall, scientists from USGS, University of Rhode Island, and NOAA discovered sea squirts invading a 6.5-square-mile area on the northern edge of Georges Bank, a highly productive area for fish and sea scallops that is 160 miles off outer Cape Cod. Sea squirts are yellow sponge-like creatures that live on the ocean floor, forming dense mats and smothering other bottom-dwelling creatures. They exude a noxious substance to ward off predators. USGS is asking fishermen in the area who suspect they have sighted this invasive to report it so that they can document the extent of the problem.