Although the dietary laws that God specified to the Hebrew people can be found in Leviticus and Deuteronomy, they actually existed long before that time.  When Noah was preparing the ark, God instructed him regarding clean and unclean creatures:

"Take with you seven pairs of all clean animals, the male and his mate; and a pair of the animals that are not clean, the male and his mate; and seven pairs of the birds of the air also, male and female, to keep their kind alive upon the face of all the earth." (Genesis 7:2-3 RSV)

"Clean" birds, animals and insects were generally those that were plant-eaters, while those "unclean" were meat or carrion eaters e.g. chickens, cattle and grasshoppers were clean, while vultures, weasels and flies were unclean.  There were some exceptions that may have been due to their being common carriers of deadly bacterial, viral or parasitical infections e.g. rats (bubonic plague), bats (rabies), and pigs (trichinosis).

"Clean" sea creatures were apparently those that were found in fresh-flowing water (e.g. bass, trout), while those unclean tended to be bottom-dwellers (e.g. shellfish, catfish, shrimp) where natural contaminants collected - a factor even more relevant in our modern world where vast quantities of chemical wastes accumulate in the mud of rivers, lakes and oceans.

A full explanation and examples listing of clean and unclean creatures can be found in Leviticus chapter 11 and Deuteronomy 14:1-21. A summary:  READ MORE . . .