The Stream Doctor

February 17th, 2003

Your questions and answers about everything stream related.

Q. Why can't nymphs such as Pteronarcys californica (Giant Stonefly) be transported from one stream to another in waters throughout the west? Some streams such as the Yellowstone have tremendous hatches and other streams not far away do not. Also, along the same line, what determines the hatch of insects--is it governed more by sunlight or water temperature or a combo?

A. Despite apparent similarities in streams in terms of basic habitat, there are significant differences in terms of microhabitat, food supplies, and basic environmental requirements that govern which species can successfully live in a given stream. If one basic constituent that a species requires (temperature range, suitable food, competition, etc.) is missing, then it doesn't matter if everything else is there - it simply can't exist where its basic requirements are not present. Now, that doesn't explain the absence of some species, such as the Giant Stonefly, in some streams and not others, but it's a pretty good bet that some vital factor is missing. There absence in a particular stream may just mean that they haven't colonized that stream.

You mention "transported" in your question. By that I infer that you are wondering why we can't physically "stock" them in streams where they are absent. I know of no instance where sufficient numbers of any aquatic insect have been moved to another stream and produced significant hatches. Now, natural colonization by flying adults and deposition of eggs into a different stream might be more likely, but still they must find suitable environmental requirements in the new stream.

As for what determines hatching, it is temperature - specifically "degree-days." Rather than going through the whole explanation, I would suggest that you open up my previous column entitled "Water temperature for hatching and trout feeding" to learn how "degree-days" works.

~ 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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