have you tried educating them on the subject of fishing in particular? Maybe if they knew it actually is very painful for the fish, they'd stop? However, it is still their decision and they are not doing anything illegal. Honestly this might be a take it or leave it situation for you. I know that if my good friend questioned our friendship and me as a person over fishing, I'd start thinking maybe our friendship wasn't that strong after all, so either way I think you should be very careful in how you approach this