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In any survival scenario, it's an uphill battle to provide enough calories to stay alive. When you're out in the wilderness and don't have access to stockpiled food reserves, you'll need to be clever to find substantial sources of food and avoid starvation.
The easiest place to start is by searching for small and slow-moving prey, like grubs, grasshoppers, shellfish, crabs, and other invertebrates. Cooking up a few handfuls of these creatures can keep you going, but for long-term survival you'll need larger meals. The next logical step is to use these invertebrates to bait a hook, and step up to fishing.
Once you've caught a fish, you could simply eat it, or you could use part of it to move up one more level in the food chain. In certain areas, a small fish can be used as bait to catch larger predatory species, further accelerating your calorie intake. In the following video, YouTube survivalist Bob Hansler shows how a fish can be used to catch snakes.
Keep in mind that this technique is most likely illegal in most jurisdictions (check your local laws), and is also inherently dangerous.
Catching snakes could provide you with a large amount of meat, or it could result in a snakebite if handled improperly. When you're in the backwoods without access to medical treatment, a venomous snakebite could easily be fatal. Even a non-venomous snakebite could lead to a dangerous infection, so if you're not comfortable with these risks, you may be better off looking for easier sources of food.