Use of Otolith Microstructure to Distinguish Wild from Hatchery‐Reared American Shad in the Susquehanna River

Hatchery production is a major component of the effort to restore American shad Alosa sapidissima in the Susquehanna River. Otoliths of all hatchery‐reared American shad are marked by immersion of larvae in a solution of tetracycline. Sagittae from juvenile outmigrants are examined for marks and the data are used to estimate hatchery contributions to the total population of the outmigrants. We have developed an alternative method of distinguishing wild from hatchery‐reared American shad based on otolith microstructure, Otoliths of hatchery‐reared shad characteristically exhibit narrow increments related to slow growth and constant rearing conditions in the hatchery, followed by wider increments formed after fish have been released into the wild. Sagittal increment widths were measured for 10 groups of American shad from known origins. Mean widths of the first 15 increments were smallest for four groups of hatchery‐reared fish, largest for four groups of wild Susquehanna River fish, and of intermediate size for two groups of wild Delaware River fish. In a blind test of the increment width criteria, we correctly classified 92% of 246 sagittae from known‐origin Susquehanna River fish of the 1986–1989 year‐classes. Applying the technique to prespawning adult American shad captured in the Conowingo Dam fish lifts, we estimated hatchery contributions to be 71% in 1989, 67% in 1990, 69% in 1991, and 76% in 1992.