Homing of Hatchery-Reared American Shad to the Lehigh River, a Tributary to the Delaware River

Abstract

Runs of the anadromous American shad Alosa sapidissima in the Lehigh River, a major tributary to the west shore of the Delaware River, were blocked by construction of dams during the 1820s. Completion of two fish ladders in 1994 opened up the lower 39 km to migratory fish. Hatchery-reared American shad larvae, marked by immersion in tetracycline antibiotics, have been stocked in the Lehigh River since 1985. From 1995 to 2000, hatchery-origin (marked) American shad made up 73–98% of the adult shad collected in the Lehigh River upriver of the first dam. The contribution of Lehigh River hatchery-marked adult shad in the Delaware River was 5–15% at Raubsville, 10 km downriver from the mouth of the Lehigh River, and 0–4% at Smithfield Beach, 53 km upstream from the mouth of the Lehigh River. These results demonstrated that American shad migrating into the Lehigh River were not a random assortment of shad from the Delaware River population. When the downstream Delaware River samples were segregated laterally on the east and west sides of the river, the sample on the west side had a significantly higher proportion of Lehigh River marks (8–15%) than the sample on the east side (2–7%), indicating that the returning Lehigh River hatchery-marked adults preferred the side of the Delaware River influenced by the Lehigh River. Our results suggest that efforts to restore American shad above dams should include some transplant strategy to expedite recolonization.