Most people have been fishing at some point in their life and in the event of TEOTWAWKI many people will include this basic survival skill in their portfolio of hunting and gathering activities. Since the majority of the world lives along coastlines, fishing for survival might become fairly competitive and with so many lines in the water you'll be better off jumping in and hunting your fish the way God intended. After all, why else would humans be given the mammalian diving reflex, the set adaptations which occur as soon as your face touches the water that maximize your oxygen efficiency and protect your organs from damage due to water pressure? So you could spearfish of course.
Spearfishing (often referred to as free diving) provides a wonderful alternative to fishing with a pole but requires a different skill set. Spearfishing is often a better approach than using a pole for a number of reasons: Spearfishing requires little to no fishing line that may be hard to come by and easily lost in the water or damaged. It does not require bait. Spears are not lost as easily as hooks. And from an ecological point of view spearfishing damages the environment less because it is more selective and leaves behind no old line to tangle up animals and trash the environment.
Despite the many positives of spearfishing there are some cons as well. Spearfishing still requires its own set of supplies that can be hard to come by in a collapse scenario. Long sharpened steel shafts with the necessary shapes and structures to lock it into the trigger mechanism will be difficult to fabricate much less find. The heavy rubber tubing required for the spear gun's power bands will also be a difficult item to find and because rubber degrades over time, the chance of finding well maintained rubber that can withstand the tension required for your purpose will be in short supply. Also, spearfishing is best done in clear water where you can visually identify your prey from a distance in order to have time to aim and fire your spear gun accurately. Since clear water typically means ocean water and much of the ocean water in North America is quite cold it requires a wetsuit and good swimming ability to accomplish comfortably and safely. If you are going after large, active fish, you will need a spear gun. For more approachable fish that tend to be more sedentary a Hawaiian sling may suffice. A sling is less likely to bring in the big fish but it requires much less equipment. The standard spear gun is basically a long crossbow. A steel shaft, sharpened at one end, sits atop a metal pipe or length of wood and nestles into a groove. At the back end of the gun is a handle and trigger mechanism. The trigger mechanism accepts the appropriately shaped spear shaft and holds it in place until the trigger is pulled. At the front of the gun one or more heavy rubber bands are passed through the body of the gun and connected into a loop by nylon cord. When the bands are pulled back the nylon strings are set into a small tab on the top of the spear shaft. The trigger mechanism holds the shaft in place under the tension. When you're read to fire, you pull the trigger, the spear is loosed and the bands send the shaft down the groove and into your target. The effective range of a spear gun with a typical two band configuration is roughly 20 feet under water.
Alternatively, the Hawaiian sling is simply a spear shaft with a rubber loop at the back end and typically three sharp metal wires at the front. You operate it by placing your index finger and thumb inside of the rubber loop and pulling back on the spear to create tension in the rubber band then release, much the same way that you launch a paperclip with a rubber band. This version has an effective range of about the length of the spear itself but would be a much easier version to fabricate if you have to go MacGyver. There are also pneumatic spear guns but I have no experience with these types nor do many people use them from my experience.
The spear gun. method of spearfishing usually requires a set of additional gear which includes a float (basically the orange flotation devices that lifeguards carry) that is towed behind the diver by a length of rope, typically a length longer than the deepest the diver would expect to dive (anywhere from 30 to 100 feet. The float often holds a dive flag which is required by law in many areas where boat traffic could present a hazard. The float also serves as a place to tie up dispatched fish while the diver hunts for more. It also keeps the dead, bloody fish away from the diver in the event a fish shows up for a free meal, especially the kind that can make a meal of the diver himself. The gun is clipped on to the end of the float line to secure it in case the diver has to drop the gun. Some very large fish can be taken with a spear gun and the diver wants the option of dropping the gun and letting the fish wear itself out against the float. Divers have lost their lives struggling to bring large fish to the surface.
A diver also wants to bring gloves to handle the spiny fish and a knife to finish off any that weren't dispatched from the initial shot. Wetsuits are a must in cold water but even in warm water become necessary as many spearfishing expeditions can last several hours, long enough to dangerously chill a diver even in the tropics. Fins are essential as they make swimming much easier, make for much deeper dives, and allow the diver to expend less energy, leading to a lower heart rate, less air consumption, and more time underwater where the fish are. A high quality mask is a must and in my opinion the most important thing to pay a high price for. There's nothing worse than dealing with a leaky, poor fitting mask while your in the water. You don't want to even think about the mask. Get one that has a wide field of view but a low volume of space between your face and the lens as this space will require air from you lungs to equalize as you descend. The bigger the mask, the less air you'll get to keep in your lungs. Freediving-specific masks are always "low-volume" masks for the reason mentioned above but they aren't always the most comfortable and don't always offer large field of views by way of their low volume. I find the single frame masks with a single lens as opposed to two or more separate lenses offer the highest field of view and most comfort. The Oceanic Shadow is my mask of choice.
A snorkel is highly recommended because it allows you to be on the surface keep your vision focused underwater on the prey or any lurking predators. Face down is also the lowest energy position for rest on the surface for recovering between dives. If the waves begin to pick up it really helps to have a tube to breathe through so you are not fighting the waves for air. A weight belt will absolutely be needed if you are using a wetsuit and also help to lower the energy required to get down on the bottom and also allow you to rest on the bottom and be still while you wait to ambush fish. Ankle weights can help to flatten out any extra buoyancy you might have on the legs. Make sure that your weight belt has a quick-release and that you know how to disconnect it in case you need to make an emergency dash for the surface. In a collapse scenario this gear can be reduced down to a mask and Hawaiian sling or even just the sling.
So now that you've got your gear, the next question is; can you swim? If the answer is no, then it's time to start learning. Are you comfortable enough in the water to not need solid support for hours at a time? Are you comfortable holding your breath? Are you comfortable not breathing through you nose or getting water inside of your mask? Obviously swimming ability is vitally important to this survival skill as is your comfort level in the water because the idea with spearfishing is not to get a workout but to maintain the lowest heart rate you can, you want to be comfortable enough to fall asleep in the water… but don't fall asleep. If you lack the comfort or swimming ability, then get in a pool and swim. Hold your breath and sit on the bottom, work your way up to a minute underwater, first in the shallow end where you can stand up if you have to breathe and then move to the deep end. In order to hold you breath for as long as possible and thereby give yourself a higher chance of success there are a few techniques that you should utilize. First, you want to lower your heart rate and oxygenate your blood as much as possible. To do this, completely relax your body and breathe through your snorkel with slow, deep breaths - completely filling and emptying your lungs each time. Just having your face in the water will induce bradycardia (part of the mammalian diving reflex) and help lower your heart rate. Use this time to enter a zen state, focus on lowering the heart rate, relaxing, whatever meditation method helps you.
One technique I've found useful is to spend ten seconds exhaling slowly, push all the air out of your lungs and then hold yourself emptied of air for two seconds. Then breathe in and completely fill your lungs over five seconds. Hold at the top for two seconds and repeat the process three or so times. Do not rapidly hyperventilate. Now take in one final deep breath and bend at the waist 90 degrees so the top of your head is pointing straight down towards the bottom and then kick your legs up straight above your body and you should sink down into the water enough for your feet to submerge and begin kicking. You may feel somewhat lightheaded at first from the high oxygen level in your brain and even have the urge to breathe again right away, but if you give it a moment you will normalize and see that you have a lot of time before you'll need a breath. Kick in long slow motions from the hip, keeping your knees more or less straight. You will need to equalize your air spaces every few feet… do not wait until you feel discomfort. Exhale a little air into your mask from your nose so that the mask does not begin to squeeze against your face and plug your nose with your fingers and exhale gently to equalize your ears. Continue to do this throughout your descent.
Depending on your body fat level, added weight on your belt, and wetsuit thickness, you will become neutrally buoyant at a depth corresponding to these factors and even negatively buoyant at deeper depths. This is a good thing because you will not have to use extra energy keeping yourself underwater and you will also be able to remain on the bottom and blend in with the substrate to stealthily ambush your prey. Focus on remaining as absolutely relaxed as possible and keeping loose any muscle that you don't need to use. Even when you begin to feel that you need a breath, you still have a long time before you actually need to breathe or loose consciousness. One concern however is that of shallow water blackout, where on your return to the surface your body does not recognize the need for oxygen because of pre-dive hyperventilation. This can be deadly and is a major reason to always dive with a partner. Your buddy should remain on the surface and continue to watch you throughout your dive, ready to pull you to the surface and hold your head above the water until you return to consciousness.
Now it's time to get your fish. Some divers use bait or flashy objects to attract fish in close enough for a kill. Without some sort of bait you will need to wait for an unwary fish to come in close enough for a shot or slowly approach your quarry without frightening it and sending it off into the blue. Fish are much smarter than you might expect, especially fish that are commonly prey to spear fishers. I have seen many parrot fish that will remain relatively uninterested and relaxed in my presence when I am snorkeling but will then quickly turn tail and dash off when they see me with a spear in hand. Some divers use masks that have reflective lenses that hide their gaze from the fish. If you are not using this type of mask, try to not focus your gaze too hard on your intended target. Fish know when they are being watched and will get uncomfortable if you show too much interest. Just relax and wait for your shot. When the fish approaches move your gun slowly and smoothly out in front of your face with both arms extended, aim down the shaft, and pull the trigger. A fish can have amazing reflexes and dodge even a well aimed shot. If you live in the tropics, the coral substrate will provide innumerable hiding places for fish, so take the time to peak into all the nooks and crannies. In Hawaii, enormous ulua are often found hiding in small caverns that one would not expect to hold such a large, powerful fish. Some fish, like the goatfish and squirrelfish in Hawaii, are very easy to approach and you can practically reach out and poke them with your spear before you pull the trigger. If free diving sounds like too much then take a stab at it (pun) with a simple Hawaiian sling. With a good eye you can catch plenty of octopus, lobsters, and sedentary fish in fairly shallow water. With some practice you should be returning to shore with a float full of fish ready for the frying pan. If you dive in the ocean, soak all of your gear in fresh water to prevent salt damage. Take the time to learn how to replace and repair parts of your gun, stock up on extra power bands, and maintain your equipment well for the day when the lights go out and don't come back on.
JWR Adds: The best way to store natural rubber items is in a cool dark place, with a coating of talcum powder.