Sturgeon were the biggest fish in Lake Superior - reaching up to 100 pounds and several feet long. Individuals could live a century.
They're rather ugly, in an attractive kind of way -- with a shark-like tail, a couple of barbs under the chin and rows of spiny bumps running the length of the fish like an armored skin.
At the St. Louis River, Daryl Peterson stands in a light snowfall, next to a spot where sturgeon might have spawned a century ago. Peterson, who works with the Nature Conservancy, thinks there's a wow factor with sturgeon.
"When you see some of these big fish come up, and they're swimming around and they're just kind of playing around in the rocks up here, it really gives you kind of a neat sense of just what an amazing fish they are, and to think about these, that they lived to be nearly 100 years old," Peterson said.
The species actually is prehistoric, dating back to the days of dinosaurs.
You can find sturgeon in other Minnesota waters, but Lake Superior only has two Minnesota rivers suitable for sturgeon spawning -- the Pigeon, on the Canadian border, and the St. Louis.
There was a time you could hardly fish Lake Superior without catching a sturgeon, but the ancient giants were all but wiped out sometime in the last century. Experts blame over-harvesting, and then damage to the rivers where sturgeon spawn.
"In the logging days there were millions of board feet that got floated down this river, and that really, really did a number on the habitat," said John Lindgren, with the Fisheries Division of the Department of Natural Resources. "That was followed by a lot of industrial abuses too, so the sturgeon population disappeared."
To reproduce, sturgeon need shallow water or pools where a single female may set a dozen male fish into a frenzy. It's an elaborate dance with the fish half out of the water that can go on for hours.
The problem is, in the St. Louis River there's a dam. This time of year the Fond Du Lac dam is a high wall of water spray and yellowish ice. The dam blocks the sturgeon from reaching the kind of shallow water in the spring, where the male sturgeons like to dance with the ladies.
If there's going to be any spawning going on, it will have to be right at the foot of the dam.
That's where the dump truck with the rocks comes in. A pile of rocks are rolling from the truck, which has snaked a narrow single-lane road through the woods of Jay Cooke State Park.
This winter, a small crew is pushing the rocks into huge piles on the riverbank. Daryl Peterson explains that in the summer, they'll drag the rocks out into the river to back up the water, creating pools.
"By placing rock across the stream, kind of stepping the water down, this will allow the water as it goes up and down with the flow of the river to provide spawning habitat kind of all across that channel," Peterson said.
Some of the boulders going into the river are as big as a compact car.
"We'll also place some of these really larger rocks in boulder complexes, just kind of randomly spaced around, and that will give the fish places to hide and rest while they're coming up getting ready to spawn," Peterson said.
The sturgeon's comeback dates to 1979. That's when a new water treatment plant went online and did a better job cleaning industrial chemicals out of the region's wastewater. The river is now remarkably cleaner.
In 1983, wildlife officials began introducing sturgeon fingerlings raised in Wisconsin hatcheries. Those fingerlings are now over four feet long and coming of age -- young fish looking to hook up.
Peterson said the sturgeon's survival is more than a fish story. It's about restoring parts of an ecosystem in the river and Lake Superior.
"These fish are slow growing," Peterson said. "They're very long lived ... so to bring them back we can really demonstrate that we're improving the habitat quality. We've really improving the water quality in western Lake Superior."
So far, the planted fish have survived well and grown. But the next step is to give the fish an opportunity to reproduce. Once this underwater dance hall is done, it will be time for the fish to tango.