The Stream Doctor

December 1st, 2003

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Q. From FTPUC: You stated that insects hatch after a given number of degree days. I believed, based on several years of temperature recording, that the Ephemerella subvaria in a local stream in MA hatched at a water temperature of 48 degrees. Was this just a coincidence?

A. I'd be willing to bet it is a coincidence. But, it is a coincidence based on the natural thermal history the stream goes through each year whereby the necessary number of degree days is accumulated at the time the stream reaches 48 degrees. Look at it this way. An insect requires a certain development time as it goes through its instars and this rate is governed by temperature. If your population of E. subvaria was, let's say, 75% through its development time and something happened that warmed the water quickly to 48 degrees, the insects would not be developmentally able to emerge. Their development would be accelerated and they would probably emerge earlier than normal (by acquiring the necessary number of degree days faster), but they still need to pass through the necessary developmental stages to reach maturity.

I'd also be willing to bet that your stream has a fairly constant temperature regime during the year - one that is not interrupted by abnormal fluctuations. This would explain why your population appears to obtain their necessary number of degree days at a fairly constant time, in terms of water temperature, each year.

I hope this helps. If you want to pursue it, contact me directly at

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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