How Does HFF Calculate Hatch Timing?

Rather than a typical “hatch chart”, HFF’s hatch timing calculations are based on accumulation of thermal units in the aquatic ecosystem. Different species hatch after accumulation of a given number of thermal units, with earlier hatching species requiring fewer and later species needing more (i.e. a species requiring 100 degree days will hatch earlier than one requiring 200 degree days). Also, warmer water temperatures allow accumulation of thermal units to happen quicker. Using thermal units as a gage allows us to predict timing compared to average.

So, we report the number of days difference between average accumulation of thermal units and what has actually happened based on water temperatures since the beginning of the aquatic growing season (approximately April 1st).

For example, when we report hatch timing is “within 1 day of average”, that means that all of the species that normally hatch around this time of year will be hatching at roughly their normal time. And “average” is based on the aquatic insect data we have, spanning 2014-2024.

Our next step is to scientifically compare these calculations with insect observations in the field, which will require a massive sampling effort and 3 or 4 years of field work. To be continued.

To view HFF’s Hatch Timing website, click below.

Hatch Timing Website

To learn more about HFF’s expansive aquatic insect monitoring program, AIM for Healthy Rivers, click below.

Insect Monitoring Program