Where have you been fishing at? There's a hidden cavern in the area, which I believe is the only place the big fish can be found. If you've already been fishing there, then the only thing you can really do is just keep catching fish that look big. IIRC, the sharks or whatever usually weigh the most.
The fish ARE difficult to catch there. As soon as you hook them, they swim as far away from you as they can. They also seem to get away more easily in this level. I recommend fishing in the war as opposed to standing on some ground.
The area around the the secret cavern has some large fish, as well.
Holy, holy Lord God Almighty. Worthy is the Lamb who was slain. Highest praises, honor, and glory be unto Your name.
Fishing is easy once you get the hang of it! Just keep in mind Big's position relative to the fish, and try to keep it directly in front of him with left and right taps on the stick. Remember that left and right always correspond to Big's left and right, meaning that when the camera is facing him, if the fish starts swimming to your left, tap left on the stick. Big will tug to his left and resist the fish. When the fish is in front of you, throw in some down taps between reels to create some slack to wind up. When the fish has an energy surge (the splashing noise), stop reeling and just resist it with tugs. Also, if the fish rams into a wall too much, it may spit out the lure, so avoid that too. Hope this helps.