I've seen stuff like that in Michigan. Those shad breed like crazy and grow fast, and they don't really seem to do that well in bad winters, so you see winter and spring die offs a lot. It's not pollution (radioactive or otherwise), because it's almost entirely limited to the shad. Bass live everywhere they do (young shad are a big prey/bait fish for largemouth bass) but you almost never see bass die off along with them.
Gladwin used to have a special once-a-year sanitation program to clean up the shores of the various lakes in the area (they might still for all I know) - Sanford Lake and Wixom Lake seemed to have shad kills almost every spring when I went there in the 90's.
Killed my sad attempts at teenage romance a couple times - finally got this one girl to at least take a walk and the beach was ankle deep in reeking dead fish.