Notes: Differences in stocks of American shad from the Columbia and Delaware rivers

American shad Alosa sapidissima from the Columbia River on the Pacific coast and the Delaware River on the Atlantic coast were reared from 3 June to 24 October 1986 in two adjacent hypalon‐lined ponds. Although fish from the Columbia River were introduced into ponds 29 d after those from the Delaware River, they grew significantly faster and attained a greater final weight. Fish from the Columbia River also had lower mortalities at all test salinities and temperatures than fish from the Delaware River. Electrophoresis revealed allelic differences between the two stocks at one locus (creatine kinase). We conclude that the two stocks of American shad are sufficiently different so that managers should not introduce them to different river systems without careful consideration.