(Oooops saw you have already done this...yes please keep us in the loop)

Like esshup says I would reach out to this guy just to ask for suggestions and so it at least starts the process of his involvement. He will tell you how best to start gathering your evidence and about placing signage. Then he might stop by if he is in the area in next few weeks.

How many "No Trespass" signs do you currently have? If you havent already, take pics of fishing poles with bobbers in truck that also shows license plate parked bext door. If they own the house look up their name on the county CAD and see if Game Warden can run their names or address to see if they have fishing licenses.....if they rent get owners name and send a letter or if you have an attorney friend...an attorney letterhead always has impact...that can send a warning letter to landlord concerning what the tenants may be doing and it would be helpful if the landlord would warn his tenants.

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Texas Park & Wildlife - Trespassing

Fishing has never been about the fish....