Degree-days is a concept that gives an idea of the amount of heat needed for the development of the embryo in the egg. It is the product of the average daily water temperature by the number of days of caviar development with this temperature. By the number of degree-days, it is possible to judge the duration of embryo development. For example, it takes 45-50 days for the development of rainbow trout eggs at the temperature of the water washing the eggs, b—10 °. The total amount of heat for this period of time will be about 360-400 degree days, or an average of 8 ° for 1 day. For the development of carp caviar, carp usually takes from 3 to 7 days at a water temperature of 18 °, i.e. the amount of heat is 54-126 degrees days.

Degree-day is a conventional unit of measurement of the excess of the average daily temperature above a given minimum ("base temperature"). It is calculated as the sum of deviations of the average daily temperature from the base for a given period of time. If the base temperature is set to 10 °C and the average daily temperature was exactly 20 °C for 30 days, then the excess for these days will be 30 days × (20 °C − 10 °C) = 300 degree-days.

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