Well, if it were my game, I think I would develop the fishing mechanics into a complete fishing game. You'd have a price for different types of fish, which could have different levels of skittishness. The player would start with a fishing rod. They'd have to decide which upgrades to purchase for it. When they're ready, they'd unlock the harpoon, the net, the hand grenades, the rocket launchers, and any other lethal weapons that could be useful against a small school of fish.