The invasive mussels came in and altered the food web. Alewives, one of salmons' favorite fish to feed on, have mostly disappeared from the Great Lakes. Without enough time, the salmon were unable to adapt their diet as their food supply disappeared. Walleye, a native species, have returned to the lakes and are also responsible for eating the salmon. Last fall the Lake Michigan salmon never showed up at spawning time. Things aren't looking good for the salmon.
Listen to the NPR Story for more details.
Photo credit: Coho salmon - muskegon-mi.gov