Thermal marking of Atlantic salmon otoliths

By exposing fry of Atlantic salmon Salmo salar to various temperature regimes, we determined conditions required to produce a readable thermal otolith mark. Marks were quantified by using a darkness index that represented the number of times darker the mark was than the background. Minimum requirements for a strong mark were 1 d at a relatively warm premark temperature of 9°C and 8 h at a lower marking temperature (1°C). Temperature decreases of 4°C (from 5°C to 1°C) resulted in strong marks if fish were held at the premark temperature for 4 d and at the marking temperature for at least 24 h.