Even if you do get pics of them fuzz aint going to accept them as legal evidence or do 'owt, and you're not allowed to lay traps or deliberately hurt them - that's official, police public meeting 'cause country folk locally up in arms. That's also why the copper got killed by the gypo kids recently, they took weapons and the householder wasn't allowed to hurt them/do anything to prevent and they knew it. But...
Set up some tents/fishing tents around the lake and leave a few lamps on inside. Carelessly leave a few bits of barbed wire or scrap rusty metal in the long grass/routes thru'.
Have you started to dig the trenches for the french drains into the lake yet, and forgot to leave the greasy plank over it?