Predicting the hatch date of salmon using temperature units (TU)

When we delivered the eggs they had accumulated approximately 756 Temperature Units (TUs) up until you received them. Fall Chinook salmon eggs need about 900 Accumulated TUs (ATUs) to begin hatching. Subtract 756 from 900. 900-756 = 144 TUs = what they have left to accumulate to hatch.

Students should record _daily_ the water temperature, daily ATUs accumulated, total ATUs to date, and expected days to hatching. Daily ATUs are 1 unit for every degree the water in the tank is over 32 degrees F for 24 hours. (For example, if the water temperature in your tank is 50 degrees F.( just used as an example) for 24 hours, then 50 – 32 = 18 degrees over 32, or 18 ATUs for that day)

You can calculate the approximate number of days until hatch by the following formula:

900 – 756 = 144 (the number left to accumulate to hatch) divided by one day’s ATUs = the number of days until hatch.

For example, when the number of ATUs when we arrived with the eggs was 756 and if your water temperature is 50 degrees F, then 900 – 756 = 144 divided by 18 ATUs/day = about 8 days until hatch.

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