In a true survival situation, staying alive becomes a battle against nature, and when you're fighting for your life, there must be no holds barred. This is why you'll sometimes hear us discuss survival methods that are discouraged, considered unsportsmanlike, or are even downright illegal under normal circumstances. The gill nets we'll be discussing here fall into the latter category in many regions — in other words, there's a good chance they are against the law where you live, so check your local regulations before trying any of the following.
Fish can provide a sustainable source of calories, protein, fats, amino acids, and other nutrients. In a survival situation, actively fishing from the shoreline or a boat with a rod and reel consumes time and effort that could be put towards other critical tasks. This is where passive methods come in. Leaving some form of trap in the water can generate a food source, even when you're not monitoring it. This is why our ancestors progressed beyond hunting and gathering — it's not exactly efficient in the long term.
One of the most effective types of passive net is the gill net. These are vertical panels of netting which are set across bodies of water, with floats along the top of the net and weights along the bottom. Once the net has been deployed, fish attempt to swim through it, and become trapped as the net snags behind their gills.
The size of the gill net's mesh determines the fish it is most effective at capturing. Small fish can pass straight through, while large fish can't fit their heads through the mesh and are able to swim away. In survival situations, this means you'll be able to capture a steady supply of fish which are large enough to provide substantial calories, assuming the location you choose for the net is viable. In the past, the high effectiveness of gill nets often led to over-fishing and damage to the ecosystem, and that's why they are so heavily regulated today — but in a SHTF scenario where you need to find food to avoid starvation, they can be lifesavers.
The following video from Bob Hansler shows how to set up and use a gill net in a river:
Regardless of the scenario you're in, use this knowledge responsibly. Even if you are in a survival situation, you don't want to decimate the local fish population with an unsupervised net, leading to more food than you can eat before it spoils. Do what you need to in order to survive, but be cautious with this highly efficient passive fishing method.