Six months ago U.S. Geological Survey scientists noted that the Chicago region is the greatest contributor of nutrients to the Dead Zone In the Gulf of Mexico. I wrote about how that came to be. When canal builders excavated the Illinois and Michigan Canal in 1848, they cut through the low divide that separates drainage to the Mississippi and the Gulf of Mexico from drainage to Lake Michigan. The divide is located several miles outside of Chicago. In 1907 the City of Chicago dredged the Chicago Sanitary Canal through the divide to carry the city’s sewage to the Illinois and Mississippi Rivers. This is how nutrients get from the Chicago region to the Gulf of Mexico.
Well, it’s a two-way street. What allows nutrients to flow south can allow Silver Carp to swim north to the Great Lakes.
JoAnn Fastoff has an excellent article in the Chicago Examiner about the need to keep Silver Carp out of the Great Lakes.
The Asian carp or silver carp or bigheaded carp is an invasive species indigenous to India and China. In 1973 fish farmers imported and stocked carp to control phytoplankton, algae, in their ponds. The phytoplanktons are microscpic plants–food for larval fish, native mussels, and zooplankton–that drift in the well-lit surface of a lake. Within a few years six state, federal, and private fish hatcheries were raising carp. By the end of th edecade municipal sewage lagoons were stocking the fish. By 1980 they had escaped into the nation’s rivers and lakes, where they reproduced and increased their range exponentially throughout the Mississippi River Basin.
The carp scoop plankton from the surface of a lake, competing with native fishes that rely on plankton for food: gizzard shad, bigmouth buffalo, and paddlefish.
Ironically, a fish that was introduced to control algae led to the production of more algae. The carp feed on algae, excrete nitrogen and phosphorous nutrients, which produce more algae. The also feed on zooplankton, reducing the number and size of plankton that would feed on algae; hence more algae and less oxygen in the waterways.
Silver carp swim in schools, just below the surface of the water, and when disturbed, jump. Noisy outboard motors upset them. They leap into the boats, often damaging them and knocking the boater silly, leaving behind slime, scales, and feces. They are set to cross the barriers between the Mississippi and the Great Lake Basins. They have reached the electric fence in the Chicago Sanitary Canal. They are in the Des Plaines River, adjacent to the Canal, and should it flood, the carp could spill over into the Canal.
Or, during a heavy rain, they could sneak through culverts that connect the Illinois and Michigan Canal to the Chicago Sanitary Canal .