Snags! [Versión en español]
There are two sorts of snags. The first is behind you. The second is in front. Both can be either above or below the water surface, although if you hook one behind you and it's below the surface then this is a good indication that you have made a serious cock-up somewhere and would be well advised to give up flyfishing and stay at home.
Karen: How can you hook it below the surface on the backcast?
Paul: Well, you could have a low backcast, the fly could land on the water, and then, during the pause, sink and hook a snag.
Karen: But what if there is no water behind you?
Paul: Then you should definitely have stayed at home.
Karen: That's your answer to everything.
Some snags are stationary. Some are not. Sheep for example would generally be regarded as 'moving snags', especially once snagged.
Karen: Here (in Australia) snags are also sausages.
Bulls are another example of a moving snag. If you hook one of these you would be well advised to drop your rod and run like hell.
Karen: And forget all about the barbie.
Working the fly free
Stationary snags are the most common. The way to deal with a stationary snag is firstly to try and ease the fly free. Point the rod directly at the obstacle and pull the line gently. Take care to apply some downwards pressure on the rod. If you are not careful, and even if you are, the fly can suddenly free and the stretch in the line will flick the fly very quickly towards you. Wear sunglasses at all times and keep your mouth shut.
If the slow stretch doesn't work, release the tension and feed some more line out. Take that line and form a D-loop, and roll cast the loop directly at the snag. This often works.
If it doesn't try the slow pull again but this time with some more 'oomph'.
Working between the roll cast and the slow pull often frees the fly. If this doesn't work you might have to walk up to the fly and release it yourself.
When this is impossible you can swim out to retrieve your fly, otherwise break it off. Point the rod directly at the snag and pull. Duck. If you are lucky you might still get your fly back. If you are unlucky it will hook you.
A very useful thing when your fly is up a tree is to use the tip of the rod as a disgorger. It works very well. Just poke the fly about with the tip of the rod. I unhook small fish this way.
Paul: But not up trees.
Oh and it sure helps if you are barbless. I always fish barbless; you hook more fish when you do.