Fish are mostly cold-blooded aquatic animals. They swim in water when alive and float on water when dead. Many animals eat fish. Several bears, for example, and otters actively hunt them. It is believed that this is where human beings got the idea from, and within period net and line-fishing were widely known, and many rivers were being developed to make this easier, by introducing fish-weirs.
As the Bible did not consider fish to be animals, the Church allowed them to be eaten on Fridays, whereas eating other animals was prohibited. For this reason, many monasteries developed artificial fish-pools so as to have a ready stock of fish for their Friday dinner tables.