As a kid,until 17, I did it all the time. I don't do it anymore though. Unless it's County or State property. But yes, I know it's wrong and you shouldn't do it.
Sometimes trespassing is a misdemeanor and other times it could be a felony. I'm not suggesting you break the law but if you do, then know the law before hand.
And I would not recommend shooting someone for fishing while trespassing unless you can clearly convince 12 people you or someone else's life was in immediate danger of being killed or seriously injured by the offender.
You can however be detained by the owner or his designee with reasonable force until the authorities arrive.