Abarim Publications' online Biblical Hebrew Dictionary
The two roots דגה (dgh) and דגן (dgn) are quite alike in form and meaning, as both express abundance. Some scholars have suggested that these two roots aren't two at all but derive from each other.
The root-verb דגה (daga) means to multiply or increase, but as such it's used only once in the Bible. In Genesis 48:16, Jacob blesses Ephraim, Manasseh and Joseph, and says: "May they increase into a multitude in the midst of the earth" (with which he probably meant the center of prominence). The derivatives of this root are much more common:
- The masculine noun דג (dag) and the feminine noun דגה (daga) both meaning fish (literally "the multitudinous"). Fish were caught with spears (Job 41:7), hooks (Isaiah 9:8) or nets (Habakkuk 1:15), but only fish with fins and scales could be eaten (Leviticus 11:9-12). Fish feel the presence of God (Ezekiel 38:20), will exist and be hunted in the new creation (Ezekiel 47:9), but it was forbidden to worship an image in the form of a fish (Deuteronomy 4:18). When Jonah was tossed over the side, a great fish (and thus not a whale) gobbled him up (Jonah 1:17).
- The denominative verb דיג (dig), meaning to fish (Jeremiah 16:16 only).
- The masculine noun דוג (dawwag) or דיג (dayyag), meaning fisherman (Isaiah 19:8, Ezekiel 47:10).
- The feminine noun דוגה (duga), meaning a fishing or a fishery (Amos 4:2 only).
The root דגן (dgn) isn't used as verb in the Bible and there is considerable doubt whether it actually existed. The nineteenth century linguists Justus Olshausen and Wilhelm Gesenius proposed that the following noun derived from דגה (daga) and essentially reflects multitudinousness, as does דג (dag), meaning fish.
The masculine noun דגן (dagan), denotes cereal crop in general. This word is used most often to express the abundance of grain or corn. It was tithed (Numbers 18:12). An abundance of it was a blessing (Deuteronomy 7:13, Ezekiel 36:29) and a shortage of it came by curse (Hosea 2:11, Joel 1:10). At times there was so much of it that storehouses were required (2 Chronicles 31:5).
Most scholars derive the names of the Babylonian deity Dagan and the Philistine deity Dagon from this noun (expressing this deity's patronage over agriculture), but some suggest that at least in Hebrew the noun was derived from the name.
Whether that's true will likely remain an unsolved mystery, but it does illustrate that both popular reflection on and poetic applications of these words most probably made use of the obvious connections between them. It's also important to note that these connections aligned fish with cereals and not with animals. A school of fish was more like a heap of grain than a herd of sheep.