Alewives, shad, blueback herring, menhaden, silversides and sand eels plumbed the river’s depths in constantly shifting schools.

Inside the gulps, Mr. Sieswerda said, are likely sizable amounts of menhaden, a small forager fish that fishermen call bunker — a veritable fish buffet.

He suggested that legislation that reduced fishing limits for Menhaden make them more prevalent and therefore available to the humpbacks to hunt.