Fish are a great source of protein and, like reptiles/amphibians they are with a few exceptions generally edible. Those of you who are fishers out there (I am one) will have your own methods and tricks. Use them, bearing in the back of your mind the number of days you've gone out and got skunked (caught nothing). When it comes to survival, it's time to lose the fair-play attitude.
Angling or hook and line is the first approach that comes to mind when thinking of fishing. Keeping a basic selection of tackle - monofilament, hooks, swivels - in your survival kit will go a long way toward helping your cause. As an aside for any travellers out there... If ever you are likely to be travelling in places where people rely on their resources for obtaining food - the Pacific, south-east Asia, Africa for example - take plenty of fish tackle as gifts. They are worth their weight in gold to local folks, wins hearts and minds (and giving gifts is just a decent thing to do) and a good indicator of their survival value. I digress.
Fish hooks can be improvised from a range of materials. really it's up to your imagination. Anything kind of hard, hook shaped (or fashionable into a hook shape) and sharpenable item can be made into a hook. These diagrams are pretty standard in any survival book so I won't labor them. However one method that is less known or used is a gorge hook so I'll explain it a bit more. This is quite clever. It is kind of like those old 1960-70's wooden toggle coat buttons. The principle is that they are baited so they lie flat along the line - say with a dough-type or loosely tied offal. The fish gulps the bait whole, the toggle swings out and Presto - the fish is hooked in the gullet. It sounds improbable, but it works. I've caught many freshwater eels this way, and it also works for catfish. Basically any ambush predator that gulps its prey, particularly in muddy waters - and there are quite a few species like this. It won't work for fussy nibbling species. You will need to experiment with the size a bit, depending on the mouth size of whatever is likely to be around.
Angling is as easy as tying a hook to some monofilament. See Knots for some of the more useful knots. Tie a weight at the end if you want the end to sink, tie a float such as a twig on the line if you need it to float. Tie the end of monofilament to a sapling as a rod if you need to extend your reach over a river bank. I think we all can figure out how to fashion a basic fishing rig! For bait, use worms, snails, insect larvae, offal.
Your chances are improved by increasing the number of hooks. This is not much angling, as shotgunning with hook and line. In its most basic form a jig is a central line - usually monofilament but it could be paracord. A weight tied to the bottom ensures it gets to where it needs to go, and the top end is either held by hand or on a rod. If using monofilament, individual loops can be tied in the monofilament using dropper loops (see Knots) and the eye of the hooks looped through the loop. Quick and easy. Alternatively individual monofilament loops can be fastened onto paracord using a prusik knot. For those of you snickering in the aisles at my illustration - my graphic artist is on vacation! Be quiet!
The hooks can be baited or, if the fish are receptive, glittering bits of foil such as those from food packets (the little blue parts on the right hand illustration. Stop laughing! I can still hear you...) can be tied to the line and hooks as a lure. Also, many fish like cover from surface vegetation. Making your own FAD (Fish Aggregation Device) and jigging underneath it can be productive. Even if fish don't bite, they can be foul-hooked by jerking the line up. At night, light from a lit branch can be used to lure fish to the surface.
If bait is available, a longline can be fashioned in a similar way with a central line, dropper loops, and the addition of a trace line secured to a hook with a blood knot. The base of the line can be tied to a convenient tree, baited, and thrown into the body of water. Not subtle, but it works.
Trapping and netting
By far your best option for catching fish is by using some form of trap. If you just 'happen' to have netting - this is great. However, you are unlikely to just happen to have 20m of monofilament net in your pack, and tying a net requires more paracord than you are likely to have on your person. Time to think outside the box, and think smart. Fish traps are simply variants on a funnel. A basket trap such as the one illustrated is easily made with sticks. Simply make a funnel or cone shaped structure, and tie it to a larger cylinder. Do one at either end if you want to be symmetrical. With bait - fish, offal - whatever comes to hand - secured in a bag of grasses or a leafy branch tied to hold the bait in the trap will catch fish, eels, and crabs or other crustaceans.
Along streams or at mid tide areas on the shore, rocks can be arranged to form funnels which progressively concentrate fishes into a holding area. fish are not very good at finding holes in barriers, but in their defense - when they do, they all bolt out. These forms of traps have been used in artisanal fisheries for millenia
If you are likely to be stuck in an area for a while, some variant on a crab trap is worth considering. The illustration from Richard Graves depicts an idealised form - it doesn't need to be this pretty. A few hoops of light, flexible, green sapling ends joined with twine or grasses, all secured by a paracord painter will do. Deployment is simple. Lay it on the bottom with some choice offal as bait - haul it up when something is swimming over or sitting on it. Easy. This will work for crabs and marron, It will even work for fish if you are gentle. Consider the vegetation above in the jig arrangement. Mooring a Fish Aggregation Device above your suspended net could bring fish around to be scooped up. Similarly, at night light can be used in the same way as for a jig.
Spearing can be effective and are useful when used in combination with herding and trapping. Spears are easily fashioned from saplings. Try to get dry, hard wood to carve and sharpen (the sharper the better) points. Bind these to the spear. 3 to 5 prongs is about right. Don't try to rely on a single super-sharp point - you won't get a thing. A fish can usually get jammed between 3-5 prongs, if you don't penetrate the flesh. More than 5 points just increases the drag when the spear hits the surface and slows it down too much. Look for fish near the surface, preferably shallow to hopefully pin it to the bottom. Your best hopes lie with just aiming for dense groups of fish at the surface and hoping for the best. Remember with light refraction, the fish will actually lie ahead of where it appears to be.