Identification of hatchery‐reared and naturally produced Atlantic salmon, Salmo salar L., juveniles based on examination of otoliths

. Atlantic salmon, Salmo salar L., parr and smolts from three Norwegian rivers were examined with respect to whether or not they had been released from a hatchery. The hatchery background was known for some released fish and could be ascertained for others from their aberrant body morphology and eroded fins. Fish released after one winter in a hatchery had opaque otoliths like the fish examined from two hatcheries. Naturally produced fish showed a distinct, seasonal growth pattern in the otoliths, alternating between opaque summer zones and hyaline winter zones. Fish released as fry showed an otolith pattern similar to that of naturally produced fish. A test revealed little discrepancy between two independent otolith readings even though the test reading was conducted without any information about the fish accompanying the otoliths. The results suggest that examination of otoliths may help distinguish between juveniles that are hatchery-reared and juveniles that are naturally produced in the river, provided that the hatchery-reared fish have a 1-year history in the hatchery prior to release.