Tons and tons of habitat restoration. You'll have to point that out to me. We're losing wetlands at an alarming rate.
As far as studies Google some. Not just for snappers but for all chelonians. One US forestry service study found that northern populations of snapping turtles can't sustain any harvest. It's too hard to recruit adult breeding members. Studies of turtles in Algonquin have said the same thing.
They haven't given them threatened status on a whim. Worldwide turtles and tortoises are headed into the crapper. They just can't survive the combination of loss of habitat and harvest.