Not really a bug, they fish for them in very narrow areas of a map (for example the small channels near the pond at akhtuba), where the fish can only go left and right, and basically all they do is also run left and right following the fish, which then stops the tension from obviously shooting up and breaking the leader. It's not at all a guaranteed catch tho as it is basically 99% luck in hoping the fish surfaces so you can bring it back and land it, not to mention breaking a ton of leaders in the meantime before you actually get lucky and catch one Never tried it myself so can't really speak from experience, but i've watched a lot of videos of it being done.