The more you play and complete quests, the more secrets and tactics you discover. Certain fish only swim at a certain depth so you need to adjust your lures accordingly. Also take note that you're not going to find a shark in shallow water.
The online section is pretty big too (and data intensive). High scores are there for every kind of fish caught to the total number of kilos collected. After about 10 hours gameplay Mobile Game Faqs had only scratched the surface of this truly remarkable game. Winner of their top award without a shadow of a doubt.