An Accumulated Thermal Unit is a unit of measurement used to describe heat over a period of time in a cumulative fashion.
Incubating salmonid eggs not only develop over time but also according to temperature. Eggs incubated at 10°C develop twice as fast as eggs incubated at 5°C. To keep track of the development within an egg, scientists use Accumulated Thermal Units.
1 ATU is equal to one degree Celsius for one day. In water of 10°C, we would accumulate 10 ATU per day; after 13 days we would have 130 ATUs. If the water was at 5°C, after 13 days we would have 65 ATUs.
