Thursday, September 11, 2008

Appearance of New Zealand mud snail in Lake Michigan underscores need for ballast water legislation

Appearance of New Zealand mud snail in Lake Michigan underscores need for ballast water legislation

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This summer Illinois-Indiana Sea Grant announced that researchers at a field station of the Illinois Natural History Survey had discovered populations of an invasive species new to Lake Michigan, the New Zealand mud snail. If the name of this creature does not strike fear into your heart, the New Zealand mud snail’s appearance is unlikely to do that, either. In fact, you have to make an effort just to see individual New Zealand mud snails, since their shells average only about 1/5th of an inch in length.

What is scary about them is that you don’t ever see just one where they become established. In other parts of the Great Lakes, including Lake Ontario, where New Zealand mud snails were first found in 1991, they occur at densities of more that 5,000 per square meter. That’s thanks to the fact that female mud snails reproduce without need for a partner. In the course of her one-year life, each mud snail gives birth to approximately 230 young that are genetically identical to her.

As is often the case with invasive species, New Zealand mud snails thrive in a wide range of conditions, and disperse readily. They are able to pass through the digestive systems of fish and birds alive, they can float long distances on their own or in mats of algae, and they can even make significant progress moving along the bed of a lake or stream, albeit at a snail’s pace.

At home in New Zealand their numbers are limited by parasites and predation, but where they have been introduced around the world their populations grow unchecked. As their numbers increase they displace native mussels and snails, as well as other aquatic invertebrates, and thereby disrupt entire ecosystems. Scientists expect that the destabilizing impact of New Zealand mud snails on Lake Michigan will be magnified because the lake is already stressed by so many other factors.

Anglers and boaters can help slow the spread of New Zealand mud snails and other aquatic nuisance species by taking care not to transport any sort of plant or animal material from one body of water to another, and by decontaminating their gear between outings. But the sad fact is, there are no effective controls for such an organism once it becomes established.

You might wonder why we couldn’t just import a predator or parasite from New Zealand to limit mud snail populations here. Unfortunately, such fixes tend to create more ecological problems than they resolve, since predators and parasites may also run amok in new environments.

What we can do in the present is support federal policy designed to prevent the unintentional movement of plants and animals from one continent to another. Like many other aquatic invasives, including the well known zebra mussel, New Zealand mud snails came to the Great Lakes in ballast water discharged by oceangoing ships. At present the U.S. House has passed and the Senate is considering a bill that would require oceangoing ships to take measures to prevent the spread of nuisance species. Such legislation is the best means we have for promoting the ecological health of the Great Lakes by stemming the tide of invasive organisms.