This is a lake sturgeon. Sturgeon have apparently been around for about 100 million years, swimming the lakes of North America while there were still dinosaurs walking around on the land. They can live to be more than 100 years old. Some even make it past 150. And they're huge; Wikipedia says they can grow to be more than nine feet long.
So, to recap: there are ancient fish living in our lake that are bigger than you and may have been alive since before the American Civil War.
There used to be tons of them in the Great Lakes, but (surprise!) once the Europeans showed up, overfishing and habitat destruction killed most of them. Now they're protected as a species of "Special Concern" (the same as polar bears) and you can't fish them anymore. (Unless, of course, those same Europeans massacred your ancestors and forcibly occupied all of their land, in which case letting you fish what you want is literally the least we can do.)
Lake sturgeon spend most of their time in the depths, feeding along the bottom, but sometimes they're seen in the shallows, especially during breeding season. And that means next time I'm down by the lake, I'm not going anywhere near the water.