Linked from
The 7 pages that link to American shad, each with the reason it gives.
AnadromyBroader topic: Its ocean-feeding, river-spawning cycle is a classic anadromous pattern.
Delaware RiverRelated: Dams and water quality changes have affected its river migration.
Susquehanna RiverRelated: Dams on the Susquehanna obstructed its historic spawning migration.
AlosaBroader topic: It is a large migratory Alosa species with a long history in Atlantic rivers.