What is Thermal Marking?
An otolith is thermally marked by creating abrupt changes in water temperature during incubation. This induces a “dark ring” in the microstructure of a fish’s otolith. Usually these rings are created by a rapid temperature decline of at least 3 Celsius followed by a cold interval of 24 to 48 hours. This disrupts normal otolith growth, and when the otolith is viewed under a microscope using transmitted light, a “dark ring” is visible. This ring contrasts sharply with the adjacent, narrow “white ring,” which results from the relatively warm portion of the thermal cycle. By planning a sequence of temperature changes, a hatchery is able to produce a pattern of dark rings in the otoliths of all fish exposed to those temperature changes.
