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.
