The Great Lakes are home to a variety of species of fish and other organisms. In recent years, overfishing caused a decline in lake trout. The drop in lake trout increased the alewife population. In response, the government introduced salmon as a predator to decrease the alewife population. This program was so successful that the salmon population rose rapidly, and the states surrounding Lake Michigan promoted 'salmon snagging'. This has been made illegal in all of the Great Lakes states except for a limited season in Illinois. Lake Michigan is now being stocked with several species of fish. However, several invader species such as lampreys, round goby, and zebra mussels threaten the native fish populations.