Maine’s annual spring alewife migration is inspiring. We created a map to help you find locations to witness this natural wonder. In many instances you can see a fish ladder or dam in Google Maps, satellite view. Alewives are anadromous (sea-run) fish that spend the majority of their lives at sea but return to freshwater to spawn. Alewives (also known as river herring) are guided by their sense of smell to migrate upstream from the ocean to rivers, streams, ponds and lakes to spawn. Runs generally occur between May 15 to June 15 depending on water temps and river flows.