Aside from the three types of fish you can catch, there is also a chance of getting some kind of garbage (shoes, cans, etc which are worthless), recipes, and some other location-specific items.
Fishing is done by going to large bodies of waters and using your Fishing Rod/Fishing Pole. You must Hold the Square button until the lure jiggles, at which point you release. The selling prices for the different kinds of fish are:
Littering can cause the chance of getting Garbage to go up, but confessing at the church will bring this back down.
Fishing on the beach during Spring or Summer can result in a Power Berry.
Back to Nature is the first game to introduce raising Fish on your farm. Any fish you catch (S,M,L) can be kept in your rucksack and brought to the pond beside your chicken coop and thrown in. It is not neccesary to feed your fish, however feeding your fish results in them multiplying and growing in size (Small to Medium, Medium to Large). Fish food is bought at the supermarket. Fish can then be taken out and shipped, given as gifts, or eaten. Some acheivements in the game (such as acquiring the Fishing Pole) are dependent upon having fish in the pond or shipping large amounts.