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Exposing a fish to different temperature regimes causes distinct rings or "thermal marks" to appear in otoliths. By manipulating temperature profiles during early incubation, unique mark patterns can be created to identify the broodyear and hatchery of origin for hatchery-raised fish. This marking procedure is 100% effective, permanent, and has little or no impact on fish health. The fish, however, must be sacrificed in order to recover a thermal marked otolith. Thermal marks are varied by changing the number of rings and/or bands, as well as the spacing within and between them. |