The Stream Doctor

November 25th, 2002

Your questions and answers about everything stream related.

Q. From Dan in Colorado: At what water temperature do insects start hatching and trout start to actively feed?

A. First, the insect part of the question. Each species of insect (and there are several thousand) requires a certain thermal history before it hatches. Further, the thermal history may vary for the same species in different geographical areas and other environmental factors, such as food supply, may also influence the developmental time of the nymph or larvae. So, you see, a definitive answer to your question is impossible.

Now that I've explained why I can't give you a good answer, I do want to tell you something about how your question could be answered. As I said, each species requires a certain thermal history of what temperatures the nymphs or larvae have experienced and for how long. Thus, if a nymph started growing for 10 days at an average water temperature of 15 degrees C, this would be 150 degree days. If this were followed by a 15 day period when the water warmed to 17 degrees C, this would be an additional 255 degree days and a total of 305 degree days for this 25 day period. Now if our hypothetical species required 4500 degree days to hatch, it would need 4195 more degree days. If the water averaged 19 degrees C the rest of the season, it would take about 221 more days, or a total of 246 days in this particular temperature regime. Thus, if we know the degree-day requirement of our favorite hatch and had access to good temperature records, we'd be able to know when to expect the hatch to start.

As for when trout start to actively feed, this, too, is difficult to put a define number on. The problem is that optimum temperature ranges for any activity, including feeding, can vary from species to species and among different populations of the same species. This is because optimum temperatures for spawning, feeding, etc. are largely determined by the temperature to which the particular population has been acclimated. A population of rainbows living in a relatively warm stream or lake will have different optimal feeding temperatures than a population acclimated to a cold environment. In general, warmer temperatures increase metabolic activities up to a point; then it adversely affects growth, movement, feeding, etc.

Thus, it is virtually impossible to answer your question with any degree of precision. I talked to a local fishery biologist to confirm the above and he agreed with me. There are data available showing the temperatures and ranges at which various species of trout grow best, preferred spawning temperatures, etc., but this doesn't address your question. Let me know ( if you want to pursue this. ~ Stream Doctor

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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