The Rock River has found itself with an unwanted visitor in the recent past; a Giant Snakehead fish, native to Asia, was discovered in the river in 2003. (1) While some say that this fish was likely the result of an aquarium owner discarding a pet that had grown to large, this 2-3 foot fish has become a damaging invasive in other areas of the country. The Snakehead is a very large predatory fish that could devastate the ecosystem of the Rock River, feeding on smaller fish and establishing itself as the new top of the food chain, introducing diseases and throwing the system out of balance. (1)

Another invasive Asian fish has recently been caught in Rockford, and authorities fear that the Asian Bighead Carp will pose a great threat to the Rock River ecosystem. (2) The Carp feeds on plankton and will compete with native fish for food and habitat. The Asian Carp eats the same diet as many native fish, and would drastically alter the ecosystem by forcing out natives if allowed to get a foothold in a niche.

Sources:

1) Snakehead Fish, Invasive Alien Species, Found in Wisconsin's Rock River, Associated Press, Janesville, WI, September 25, 2003. http://www.flmnh.ufl.edu/fish/innews/snakeheadwis2003.html

2) Gazra, Jesse, Invasive Fish Could Threaten Watershed, Milwaukee Journal Sentinel, Milwaukee, WI, August 23, 2005. http://www.flmnh.ufl.edu/fish/InNews/invasivefish2005.html