The same button for casting is what you use to catch. You throw the lure out and as soon as you see the exclamation point above your characters head, you catch the fish. Once that happens you will see "HIT" appear on the screen, and a meter shows up. After that it is a bit trickier, there is a meter with a fish on it, and a green box. You have to keep the green box on the fish for a little while by once again using your catch/cast button