A look at alewives and salmon in the Great Lakes...

Alewives are small, silvery ocean fish that entered the Great Lakes by traveling down the St. Lawrence Seaway from the Atlantic Ocean. They grew in burgeoning numbers in the late 1960s when their main predators, lake trout, were nearly wiped out entirely by overfishing and by predation from another invasive species, the sea lamprey. At one point, alewives grew in such numbers that other fish species together made up only about 10 percent of all the fish in the lakes.

Coho and chinook salmon were introduced to the lakes by fisheries managers about 40 years ago to reduce alewife populations.

The population of alewives virtually collapsed relatively suddenly in the lakes in 2003, surprising even fisheries researchers. The reason is now thought to be a natural reaction to population overgrowth and loss of their food supply to competing zebra and quagga mussels.

The crash of alewife populations left salmon struggling to find a meal in lakes Michigan and Huron, leading to the recent decline in the number and condition of salmon in the lakes.