OK, so everyone who's trying to find the 1000 lb hammerhead or the 3000 lb great white, just relax for a bit. The weekly report says there will be a couple new flopping fish, and if you look in the banner, it looks like on the left side there's a book that says Finding Bigger Fish. I'd bet it's a guide book like the ones for finding talismans that will make big fish spawn more often.
Some people have been saying that it's not important to catch them all because it only gets you money, but NLT has said that he plans on having them unlock bonus content, so I would definitely try to get them. Maybe just wait for the next update to make it easier.