The Stream Doctor

February 9th, 2004

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

Q. From MAYFLY 4: Many of our Western tailwater fisheries present unusual conditions where fish grow very large, very fast. Is the sheer number of insects present (midges, mayflies, etc.) enough to sustain the size of the larger fish? In other words, what do the big fish eat?

A. The key to this growth doesn't have as much to do with the large numbers of insects present, but with the fact that if environmental conditions remain essentially constant, as they often do in tailwaters, then the fish can continue to grow. In a normal stream, growth is cyclical depending on water temperatures. When the water becomes cold, movement, feeding, and metabolic activities slow down; the reverse happens when waters warm up and this is the time these populations grow. In tailwaters, the populations are usually acclimated to fairly constant water conditions throughout the year and thus can continue to grow as long as there is sufficient food. Fortunately, the same thing happens to the insects. Many midge and other insect species can continue to produce cohorts throughout the year if water temperatures do not go to low; thus, a continued supply of food for the fish. Thus, it isn't so much a case of large numbers of insects, it is the fact that their production rate is high and population turnover rapid.

You might want to refer to previous columns in reference to predicting insect hatches. In constant temperature waters, especially if they are on the warm side, an insect can attain the requisite number of degree days quicker that under natural conditions and crank out more cohorts per year.

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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