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Furry crabs, twining vines and other threats

September 6, 2007
The Altoona Mirror
By John R. Wennersten


Two years ago a watermen checking his crab pots in the mouth of Patapsco River found an interesting creature. In his pot was a mature, male, furry crab that he had never seen before. Rather than toss it back into the water, he took the crab home and preserved it in his freezer so that scientists could have a look at it. Identified as a Chinese Mitten Crab, (Eriocheir sinensis) this species was native to the Far East and probably arrived in Chesapeake waters in ship ballast.

Since that first discovery, about seven mitten crabs have been found in the bay. Though few in numbers, this invasive species gives bay experts plenty to worry about. Mitten crabs, in addition to being explosive breeders under certain conditions, are very aggressive and compete with native crabs for the same food. The crabs bury into banks and levees to make their homes and contribute to soil erosion. They also like to feed on small oysters.

 According to the Maryland Department of Natural Resources, the Chinese Mitten Crab is “catadromous” – at various times of its life it can live in both fresh and salt water. While it is easy to dismiss this furry creature as a sea-borne novelty, it has the potential to grow into the same kind of invasive species problem as zebra mussels. Currently in the Great Lakes and upper Susquehanna, zebra mussels multiply exponentially and pose a threat to everything from fishery diversity to the clogging of intake valves in water and power plants.

Invasive species seem to be everywhere these days. All you have to do is visit a park or forest to see how “mile a minute” vines, Japanese knotweed and garlic mustard are conquering habitats once claimed by a diversity of native species.

Take the case of kudzu,. This form of honeysuckle from Asia was introduced in the American south in the 1930s to control soil erosion. Today the massive spread of kudzu is dominating the southern watershed and killing off our diverse forests.

According to Charles Groat, director of the U.S. Geological Survey, the spread of invasive plants and animals constitutes “one of the most serious ecological problems facing our nation in the 21st century, second only to habitat destruction.” Groat argues that many of these invasives cause losses in agriculture, fisheries, and other resource production systems. Some are vectors or carriers of diseases that affect humans.

The Mid-Atlantic region is under assault from everything from mute swans, which destroy underwater grass beds in the bay, to veined Rapa whelks, a baseball-sized snail from the Sea of Japan that feasts on clams and oysters. Hundreds of Asian snakehead fish have invaded the Potomac River and its tributaries. They were probably deposited in the river by disenchanted aquarium-owners. Recently park rangers drained a lily pond at Kenilworth Gardens in Washington, D.C. and found two adults and 5,000 baby snakeheads.

Today species from as far away as the Yellow Sea of China and the Black Sea-Caspian basin ride in ships’ bilge to new homes in Chesapeake and Delaware Bay. Bulk carriers in Chesapeake Bay discharge annually over 1.5 billion gallons of ballast water in harbors and coastal areas. More than 150 non-native species have been identified in the Chesapeake region.

Federal laws, like the National Invasive Species Act of 1996, have been increasingly tightened to monitor ballast water and control introduction of exotic species. Because invasives have a profound effect on vital sectors of our economy, former President Bill Clinton signed an Executive Order “to prevent the introduction of invasive species to minimize the economic, ecological and human health impacts that invasive species cause.”

Controlling invasives is hard work in a country increasingly connected to global trade networks. Ultimately as nations grapple with the problems and consequences of invasive species, the transport and introduction of new plants and animals will come under stricter regulation.

The best and most cost-effective solution to combating invasive species is to prevent the introduction of alien pests in the first place. To control invasive plant and animal species, scientists now are using a method called “adaptive management.” This is a management approach that emphasizes realistic targets for dealing with invasives through regulated application of pesticides, cutting back plants and animal trapping. When invasions of exotic species like the mitten crab are small, eradication is fairly easy.

Meanwhile, the easiest strategies to control invasives are often the most important. For example, don’t release aquarium fish and plants into the wild. Help educate others about this threat. And, volunteer to beomce a “weed warrior” at parks and recreation areas to help remove invasives and restore native ecosystems.


John R. Wennersten is the author of numerous books on the Chesapeake Bay and regional environments in the Mid-Atlantic. His most recent work is Chesapeake Bay: An Environmental Biography.


Distributed by Bay Journal News Service

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