Had to do some work on one of the Upstream Flood Control Projects that I manage in my county, and just had to stand there and watch while the fish were going crazy!
These small lakes (10-30ac. surface area) are Federal/State funded projects on private land. The Conservation District I manage has an easement for operation and maintenance but I have never asked permission to fish in any of them (I do all my hunting and fishing on private land in the next county to avoid any perception of... whatever people perceive).
For the 2 hours I was there, scads of two inch long, bright yellow mayflies were hatching and the fish were going nuts! Huge sunfish doing tail-over-tin-cups cartwheels out of the water. Occasionally I would see what looked like a toilet flush along the feeding lines. It was little eaters getting eaten, of course. I even had a 6 weight in the truck ready to go.
The lake was way off the county road, no one in sight. Heck, the landowner probably wouldn't even care (or ever know for that matter) if I fished for half an hour. BUT... Dad always said that being ethical was doing the right thing, even when nobody is looking.
It was still painful to have to stand there and just watch.