Wish I could wire it for a siren sound, too, but that might be overkill.
If you want a sound alarm that is motion activated, Harbor Freight makes a cheap, effective unit for $17.
I usually set one up in my driveway in town for the guys that wander up to try the garage doors and the parked cars.
It has two volume settings, and the HI setting is pretty loud and would certainly startle guys walking in the dark. However, it is much quieter than a car alarm, and would not bother the neighbors.
The small unit in the linked picture is the motion sensor. I put a piece of black electrical tape over the little red light that is below the sensor, so the bad guys cannot easily find the unit.
The large unit easily picks up the signal 50' away inside my brick house. (This portion is for indoor usage, you will have to create some type of housing to utilize it outdoors.) Both units are battery operated, so you do not need AC to place the system. The maximum range is well over 50', but I do not know the effective limit.
My system will trigger on a car going up the driveway, or a person walking within about 20-25' of the motion sensor. The alarm unit makes a loud "bing-bong" sound.
It will certainly give you a "false alarm" for a deer, and perhaps down to the size of a large raccoon on a cold night? I believe my unit has even triggered on a cat under ideal detection conditions.
Are people accessing your property only by going up the road? If so, you could put the motion detector past your neighbor's house and the alarm unit inside his house. He could then peek out his window and call the game warden. (I don't know if the geometry required for that approach would work in your situation?)
If it does work, it will be a pain in your neighbor's butt due to wildlife false alarms. Perhaps offer them a $500 reward if they call the warden and that results in a capture?
If you do buy a unit, be sure to experiment with it around your house in town. You will quickly get a feel for how it operates, and how that might be put to best use at your absentee pond.
Good luck!
P.S. My initial advice was to suggest installing a few Claymore land mines, but then I remembered that you are a far better man than I am, so I went to Plan B!
