If you tap on your device when the needle is in the green, the fish moves forward.  If you miss that green, the fish moves backwards, BUT every time you miss, the fish moves backwards less.  After 5 misses, the fish no longer moves backwards.  You can actually fish by just tapping the screen.  Every hit advances the fish, every miss does nothing after 5.  Eventually the fish will make it to the end.  You just need to stop tapping to have it finish.  

When I don't have the focus, I just do a 2 finger tap or 4 finger roll for 15-20 seconds and when I stop, there is a fish ready to sell or fry.