Genesis 7: 8-9 “Of clean animals, of animals that are unclean, of birds, and of everything that creeps on the earth, two by two they went into the ark to Noah, male and female, as God had commanded Noah.”
God reveals which animals – including fish and birds – are suitable and unsuitable for human consumption in Leviticus 11 and Deuteronomy 14. Although the lists aren't exhaustive, He reveals guidelines for recognizing animals that are acceptable for food.
God states that cud-chewing (to chew food that has been swallowed and brought up again from the stomach to chew again) animals with split hooves can be eaten (Leviticus 11:3, Deuteronomy 14:6, “ You may eat any animal that has completely split hooves and chews the cud…”). These specifically include the cattle, sheep, goat, deer and gazelle families (Deuteronomy 14:4-5). He also lists such animals as camels, rabbits and pigs as being unclean, or unfit to eat (Leviticus 11:4-8) God later includes "creeping things" as moles, mice and lizards as unfit to eat (Leviticus 11:29-31). Leviticus 11:27 continues to proscribe more animals as unclean, “And whatsoever goeth upon his paws, among all manner of beasts that go on all four, those are unclean unto you: whoso toucheth their carcass shall be unclean until the even.”
God tells us that salt and freshwater fish with fins and scales may be eaten (Leviticus 11:9-12) but water creatures without those characteristics (catfish, lobsters, crabs, shrimp, mussels, clams, oysters, squid, octopi, etc.) should not be eaten.
There are birds and other flying creatures that are unclean for consumption (Leviticus 11:13-19) but fowl such as chickens, turkeys and pheasants are not on the unclean list and therefore can be eaten. Insects, with the exception of locusts, crickets and grasshoppers, are also listed as unclean (Leviticus 11:20-23 )
Why does God identify some animals as suitable for human consumption and others as unsuitable? God didn't give laws to arbitrarily assert control over people. He gave His laws (including those of which meats are clean or unclean) "that it might be well" with those who seek to obey Him. Deuteronomy 5:29, “O that there were such an heart in them, that they would fear me, and keep all my commandments always, that it might be well with them, and with their children forever!”
Although God did not reveal the specific reasons some animals may be eaten and others must be avoided, we can make generalized conclusions based on the animals included in the two categories.
In listing the animals that should not be eaten, God forbids the consumption of scavengers and carrion eaters, which devour other animals for their food.
Animals such as pigs, bears, vultures and raptors can eat (and thrive) on decaying flesh. Predatory animals such as wolves, lions, leopards and cheetahs most often prey on the weakest (and at times the diseased) in animal herds.
When it comes to sea creatures, bottom dwellers such as lobsters and crabs scavenge for dead animals on the sea floor. Shellfish such as oysters, clams and mussels similarly consume decaying organic matter that sinks to the sea floor, including sewage.
A common denominator of many of the animals God designates as unclean is that they routinely eat flesh that would sicken or kill human beings. When we eat such animals we partake of a food chain that includes things harmful to people.