For reasons that I'm not too sure of, I've always felt that if you build it, they will come. Crayfish, that is.

Put out a bunch of flat rocks in water 1' deep near the shores of your pond (I mean lake, at 20 acres).

Wait a week and then go see if you have crayfish or not.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."