Temperature Triggers for Emergence - Fact or Fiction

I recently read an interesting article in the latest Fly Fisherman magazine, it was actually in their "Tight Lines" section which is letters from the readers. The letter was titled, "Temperature Triggers" and was sent in by C.E. Cushing PH.D. a retired stream ecologist.

In the letter he commented that he had noticed that in many magazine articles, he often reads phrases such as "triggers the hatch" and "when the water temperature reaches," all referring to specific temperature ranges that cause insects to hatch. He stated that these statements give the impression that specific water temperatures or ranges are the causative factor in emergence. Though not unrelated, this is only part of the story.

The rest of this story will be quoted verbatim.



"All aquatic insects require a certain thermal history to mature from egg to adult. This thermal history is measured in degree days, where a degree day is defined as the average water temperature during a 24 hour period. Here's an example of how it works: Let's say a mayfly deposits eggs when the water is 15 degrees Celsius. At the end of 24 hours, the eggs have accumulated 15 degree days. For simplicity, let's say the water temperature averaged 14 degrees for the next 20 days; that would be an additional 280 degree days. As the water cools down as winter approaches, we'll say that the water temperature averages 10 degrees for the next 60 days, providing am additional 600 degree days for a total of 895 degree days thus far into its development. This continues until the immature insect has accumulated the required number of degree days to mature into an adult, then it emerges. The number of degree days required by each insect group varies both with species and geographic location. Actual values are know for only a few species, and determining these values is tedious work.

The point here is that insects do not necessarily emerge when the water reaches a specific temperature. It only means that they have acquired their necessary number of degree days to complete their development.

This doesn't invalidate angler observations that a certain hatch often come off at a certain temperature. What it means is that in a normal average year when temperatures reach that magic temperature, the necessary thermal history has also been attained. If water temperatures for a particular year have been below average, hatching times will be delayed. . .

. . . As a retired stream ecologist, I've been "Jousting at this windmill" with anglers for some time, and find that deep-seated beliefs are hard to change. I hope this explanation convinces a wider audience of anglers that absolute water temperatures in relation to insect emergence are relative and not causative."

An interesting thought to ponder, I thought so anyway!