That's too bad.

I would reach out, privately, to either the police or the fish/game warden and describe the problem, leaving out the actual location of the land. Once you know what law enforcement is willing to do, should you request aid, then I would be in an evidence gathering mode.

As I'm an absentee land owner too, I've found my most 'fruitful' evidence gathering trips have been at times when I wouldn't normally be at my pond, such as weekdays.

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."