UPDATE: Probably natural causes, river advocacy group says.
Michael Marshall went out for his usual afternoon jog in the Mystic River Reservation just upriver from Assembly Row - and was struck by the sight of "dozens, if not hundreds, of these large fish. All dead."
He says he reported the fish kill to DCR, adds: "There was also this strange orange boat with wheels churning the water. I wondered if that had anything to do with it."
Ed. note: The original headline for this post mentioned a "mysterious" boat churning through the water. As you'll see in the comments, it's a weed chopper and harvester.