Crayfish have been around for 115 million years or so. They prefer clean, oxygenated, warm water. They're most active at night. They move to deeper water and become torpid in winter. They're like canaries in the a coal mine, in that they don't tolerate pollution or low oxygen levels. You can judge the quality of water by the crayfish in it, like you can judge the health of a garden's soil by its worms.
This is a very expensive welded-trap that gets a lot of hype on the internet. They do catch very well, probably better than any others I've tried, but unfortunately they rust away quickly. Maybe there's something corrosive about our New Hampshire water.