Tuna and salmon, for example, do not originally live to be food for humans. They are called "food" because humans eat what happens to be alive on the earth. This applies not only to fish but also to animals and vegetables.
However, some animals are modified or bred for human consumption. Bovines, which are used to make beef, are all bred by humans for that purpose, and vegetables such as radishes and onions are also grown for human consumption.
In this way, we can see that all food was originally found on the earth in the beginning. Humans later modified it to suit their own needs. Things like chocolate and ice cream, which did not exist in the first place, were invented by humans by combining various ingredients.