Salmon were introduced into Lake Michigan in the early 1970s to eat alewives, a type of small shad or herring, that died off in huge, smelly numbers every year.
From about 1975 to 1977, one of my friend's dad had a boat that was docked in Waukegan Harbor. We often went up and trolled for salmon. We caught some on every trip and would often grill them dockside, just delicious.
I've been gone from the Midwest for almost 40 years but read that the alewife population is in serious decline perhaps due to some invasive mussel species. I'm not sure how the salmon population is holding up.