I'm in the military and put in 3.5 acre pond on my farm 250 miles from where I am currently stationed. It is stocked with bass, bream, and bluegill. I've often wondered what I would do to keep the trespassers away. This is the idea I came up with:
Warning! Mercury contamination. Do not touch or eat fish from this site.
I got the idea from a guy who puts up signs to deter deer dog runners that say "Warning! Coyote bait out."