Definition of "alewives"

Alewives are anadromous fish: Born in freshwater, they spend their lives in the ocean, returning annually to their birthplaces to spawn. Until colonial-era dams cut off their migration, hundreds of thousands of alewives would have come pouring into Rogers Lake [Connecticut, USA] every spring – and into other lakes like it along much of the Eastern Seaboard. Farmers used to apply them to their fields as fertilizer, and all along the coast, river herring festivals celebrated their arrival.

2014 April 20, Richard Conniff, “An evolutionary family drama”, in The New York Times‎