Thermal Units

[JOE.T]

Thermal Units aka Degree Days is a method used to measure the growth of Aquatic nymphs/pupa.Thermal units are like points.After a certain amount of thermal units are accumulated the nymph/pupa transforms to an adult.

A mayfly nymph grows to maturity but not at the same rate every day or month.For the purpose of explaining the principal of thermal units I will refer to the mayfly as x and use "round numbers" to keep it simple.


X lays her eggs June 1.The eggs now begin to grow.X needs 2000 thermal units before emerging as an adult.The avg water temp is 67 deg that day and the eggs accumulate 20 thermal units.July 17 avg water temp is 71 deg the nymph accumulates 30 thermal units that day.Keep in mind the units are be acumulated daily and being totaled as the nymph grows.Sept 25 avg water temp that day is 54 deg only 10 theraml units accumulated that day.Nov 12 avg water temp is 46 deg only 5 units that day.Dec 12 33 deg water temp 0 thermal units that day!

Ok so during the winter especially like the previous two the nymphs pretty much accumulate no thermal units since it is so cold there metabolism drops dramatically and they do not eat or grow.

Now its the spring and water temps start to rise and once again the nymphs start to accumulate thermal units.With the warming water x will finally reach 2000 themal untits and hatch.


If you look at 3 winters ago winter you can now understand and see why hatches came early and very early in some cases.There were many thermal units accumulated that winter.