Fishing as a food source is a useful skill in some parts of the world. If you know what you are doing you might be able to supplement your protein intake with some lightweight gear. Forget about hobbyist fly fishing gear, we want the food.
Watch out for fish and game cops, they can take all of your gear, car, or boat as a punishment leaving you destitute if you are accused of poaching or using illegal non-sporting fishing methods. If you're really heavy into fishing, a license and state fishing rules book can usually be had for around $5 over the counter at any sporting goods store, and it's better safe than sorry. Look up the local laws on fishing so you know how you are legally permitted to fish and how to identify the fish it is legal to catch! Your "friendly" local ranger can now carry a gun, tazer and pepper spray, so ask yourself if saving the money is worth it in the face of police brutality.
In over fished areas forget about wasting your time. Sport fishers, hunters and gatherers are often out in huge numbers during hard times and will often cause many years of damage from taking too much from the ecosystem.
Fishing licenses are usually cheap too, but you can get away without if you stay away from heavily trafficked lakes or streams. Fish is one of the most nutritious meats around, and all you have to do is scale the fish and cut out the guts (Which can be used for bait, as well) to eat that fresh, slippery sucker.
Fish can be kept in a rocked off area of the river for future eating, or for fattening them up. If you're lucky enough to get a hold of turtle eggs, I'd recommend leaving them where they are, and coming back for them when they're little and hatched. Depending on how long you're out there, you could raise them up just like regular farm animals, and eat them! The shell makes a great pressure cooker, with them in it!
Cheap fishing gear can be found in any pawn shop. Bobbers, hooks and sinkers all sell for under a dollar at most sporting goods stores, but they're usually small enough to steal for the really cheap. Dig up worms in a rainstorm, throw them in a Styrofoam cup with a soil and cornmeal mix and cast away.
A telescoping pole and spinning reel will do the trick without breaking your bank or back but a stick and floss will work in a pinch. Some six pound test monofilament line will bag most small river fish, but as said before, a pawn shop can hold a treasure trove of high quality fishing gear. Estate or garage sales are also worth a look.
A fishing rod is an important, yet durable tool for fishing. There are many different types and "actions" (strength) of rod. Stronger action rods are more suited to larger fish, yet are less sensitive to bites. That said, a medium-light action rod can catch most freshwater fish and anything too large for the rod will break the line first. Most rods are graphite these days, although fiberglass is stronger and more sensitive. Like most fishing gear, rods are being updated with all sorts of totally useless features designed to fleece the rich out of their cash, so you can often find "last year's" models on sale for cheap.
A good reel is a good investment. The third world made models will break down fairly quickly, but a good reel will last a lifetime. The two main types of reel are spinning and bait casting. Spinning reels are more versatile and easy to use, whereas bait casting reels are generally heavy duty, but harder to cast and use. As a rule, good reels are made outside of China, and can be found in pawn shops. You can also find good deals on discontinued, but perfectly good reels in many sporting goods stores. Many of these are just "outdated" models, which will still perform just fine and last as long as you want to keep them.
Ask locals their favorites but in general worms, corn, insects, and dough balls all work good, it won't hurt to carry some flashy, rattly, and spinny lures with your gear. These can easily and cheaply be made. Walleye and pike fishermen throughout the world use a simple lure called a crawler harness, which consists of a spinner blade (Cheap, flashy stamped metal in a teardrop shape with a hole on the top for the line), colored beads and 2 hooks, allowing the use of a whole worm. Add some sinkers and you have a cheap, effective lure. If you have a little money, the supplies for making lures can be very cheaply obtained. Get a big assortment pack of fish hooks, they weigh and cost very little and are useful for a hobo on the move even if you just use a stick pole and dental floss. A bobber hangs your bait off of the bottom in still water, moving water requires casting out and recovering as it floats downstream, you can use a dry stick as a bobber if needed. For simple hook and bobber fishing, you'll also need sinkers, which are simple lead weights that go on between the hook and bobber. They are also very cheap, and you should get an assortment. Bigger weights make the bait sink faster and deeper, whereas smaller weights can be used to suspend it slightly off the bottom.
A small net at the end of a large triangle of rocks laid in the river to guide the fish in will make a good trap. A series of baited hooks in the water strung to tree branches above, the fish will stay alive as long as they are still underwater. Check your fish traps regularly as they are at risk from poaching by raccoons. Spear fish in the shallows as you are setting up your nets or hook traps with a barbed frog spear tip on a stick.
Bombing fish in a pond with chlorine bleach is classless destruction, it will bring fish to the surface but will often kill the whole ecosystem. Instead use rotenone from the organic plant store or crush green husks from butternuts or black walnuts. Throw the husks into still or slow moving water, it will do the trick just stunning the fish without killing the other animals, collect and eat or salt and smoke all fish you stun.
Although harder than other methods, fish tickling is a common method of catching fish without any equipment. It involves getting the fish you want to under a rock, or some other natural barrier it can't back out of, and slowly moving your hand towards it until you can tickle its belly. The fish will become relaxed to a point where you can grab it and throw it onto the ground or catch it in a net.
If you catch a fish it should have firm and elastic flesh, clear and full eyes, bright red gills, a clean pleasant odor, and an absence of reddish discoloration on the ventral side of the backbone, i.e. the side of the backbone that's on the inside of the fish. Cloudy, sunken eyes and gray colored gills are the first recognizable signs of old, decaying fish. When the head, gills, and backbone are gone, like at a store, rely on your sense of smell and touch. Worn out or dying fish have much less nutritional value but cooking them will remove danger of illness and parasites.
To clean a fish smash the brain area to kill it, then cut from the anus to the throat, remove guts(these are good fish bait, and can also be used to trap minnows and crayfish), and use a dull knife to scrape off the scales. Some fish are best filleted from the ribs but small ones are easier to leave in and remove when you eat it. Pan-fry most fish with a little oil and spices, a favorite is to use banana or other large leaves with a pleasant or neutral flavor to wrap the fish for roasting, if possible rub the fish with oil, lemon and/or pepper is nice for flavor. Salt water and a smoke pit is a way to preserve fish for a longer time, even without salt a dry smoked fish will keep for a few weeks. Another cheap and tasty way of preparing fish is dredging fillets in egg, flour then bread crumbs, seasoning with salt, pepper and paprika, and frying With all cooking methods remember fish cooks quickly like eggs, over cook it and it will get tough. Pike and similar fish contain lots of small bones, so be sure to bone the meat before cooking.