Apparently a borrowing from Old Frisian hareng (“herring”) (itself maybe from here (“swarm, school of fish”), i.e. originally “school of fish” > “fish that swims in schools”), with loss of the initial syllable (*hareng > *renge > reņģe). This loandword is already found in 16th-century family names, and later in 17th-century dictionaries.
reņģe f (5th declension)
- herring (many fish species of the family Clupeidae, especially Clupea harengus membras, which is about 25 cm long, with a bluish green back and silvery sides)
- žāvēta reņģe — dried, smoked herring
reņģes tomātu mērcē ― herrings in tomato sauce
veikalā viņš bija ieraudzījis un nopircis svaigas reņģes ― in the store he had found and bought fresh herring