There is a huge illegal immigrant problem in this country and it doesn't involve human beings. Marine organisms brought to our shores by ships from overseas are wreaking havoc with our inland freshwaters.
While some attach themselves to the hulls, the majority travel in the ships' ballast tanks, which carry large quantities of water for stability when not loaded with cargo. On arrival the water is pumped overboard, often releasing tens of thousands of opportunistic organisms from overseas.
Transport of foreign plant and animal species by ships is nothing new. As long as vessels have crossed the seas bioinvaders have traveled as unwelcome passengers. Since the 1950s international shipping trade has greatly increased such that today 97 percent of world trade is carried by some 33,000 large cargo ships. Opportunities for these illegal aliens are better than ever.
As the world's largest trading nation, the United States has been particularly affected. It was in the Great Lakes in the late 1980s that the problem of invasive species first gained major public attention. Today, more than 185 non-native species live in the Great Lakes, with a new one arriving every eight months.
The zebra mussel is a primary culprit, and its story makes an excellent case study. This very successful foreigner originated in the drainage basins of the Black, Caspian, and Aral seas. First reports of zebra mussels in North American waters came in 1988 from Lake St. Clair, connecting Lake Huron with Lake Erie. No effective remediation was taken, and very soon colonies formed throughout the Great Lakes.
Today, damage is profound and not easily remedied. Water intakes and outfalls are severely restricted, fishnets are clogged, and fixed structures overgrown by colonies that can reach numbers as high as several hundred per square foot. Like most shellfish, these are filter feeders. Their filtering does clean the waters, but it also removes important nutrients needed for fish stocks and other indigenous species. It is estimated that three decades of zebra mussel infestation have cost the United States and Canada more than $1 billion in economic losses.
The mussels have migrated out of the lakes and down the Mississippi as far as New Orleans. They have spread into rivers and streams connecting with the lakes and are found in at least 20 states.
Zebras have a slower spreading cousin, the quagga mussel, also colonizing in the lakes. Since 2007, they have been found in Southern California and Arizona along the Colorado River. Clearly ships did not infect the Colorado. Instead it was pleasure boats and sport fishermen that transported these aliens.
The illustration shows the spread of zebras and quaggas in the United States and areas at risk for future invasion. This massive movement has happened in less than 30 years, but a tiny sliver of time in nature's calendar.
On a global scale, infestation by alien species adversely affects virtually all maritime nations. Each year thousands of invasive organisms are transported by ships, many thriving in new parts of the world. The economic cost of these aliens is enormous.
Both the UN's International Maritime Organization and the U.S. government have continuing efforts to develop standards, regulations, and laws to mitigate the problem. It is very doubtful, however, that the problem can be completely eliminated. As one expert said, "Invasive species are somewhat like a computer virus, easy to get but impossible to remove."
There are a variety of operational and technical means being tested to prevent further importation of these aliens. They range from adding chemicals to ballast tanks to filtration of the tank water using devices that generate chlorine, UV radiation, or ozone. The simplest scheme is to pump out ballast water at sea before entering port. Most of the organisms are freshwater animals and flushing the tanks with seawater should kill them. A more effective technique is to refill the tanks with saltwater rather than just flushing them.
Any system chosen for on board cleanups will represent added operating costs to ship owners. An alternative is to ban dirty vessels from visiting our ports. That's a poor idea, but it creates an incentive for the ship operators to invest in necessary ballast clean-up equipment.
On land or at sea, the problem of introduced non-native species is a very old one. For as long as people have moved around the world they deliberately or accidentally carried with them species that thrived in their new homes, most often at the expense of native species that were driven out. In the United States the annual economic and clean-up costs of invasive species is in the order of $138 billion.
Perhaps someone can develop a recipe for cooking zebra mussels?