Definition of "alewife"

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

I saw in Cohasset, separated from the sea only by a narrow beach, a handsome but shallow lake of some four hundred acres […] , and, after the alewives had passed into it, it had stopped up its outlet, and now the alewives were dying by thousands, and the inhabitants were apprehending a pestilence as the water evaporated.

1865, Henry David Thoreau, Cape Cod, Chapter I. "The Shipwreck", page 14.