Trespassing on Neighbor's Land for Pond & National Forest Access Question

[Matt M.]

My family has owned a cabin on 40 acres in Boulder County, Colorado of 60+ years. We have always fished the neighbors pond and walked through their land on the way into the national forest. Our stream feeds the pond, and we own right up to where they meet. The neighbor's land was just sold to a new owner who has already told some cabin guests of ours to stop fishing the pond. 

With that being said, since we have had access all along, is there some legal ground we can stand on to keep it? Some kind of easement, etc?