The Stream Doctor

March 21st, 2005

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


Q. I read about the in pool insect drift occurring about 1 hour after dark and again early morning before first light. Both times the insects were reported to crawl back to their niche areas. I would like to get your theory about the purpose of the drift since the material I read indicated that the drift was still under consideration as to purpose and effect.

The reference material is located at "http://www.cffcm.org/streameco/streameco-sec2-drifting.htm"

I have had some luck with the Ku fish since we talked. I also located the Kashe Stream (nicknamed the Crystal Palace of the Chuoshui River) that contains rainbows. Best regards, Mike aka: Rollcaster

A. Hi Mike. I'm going to do several things in response to your submission, including some rebuttal to the article on pool drift that you cite above.

First, I'd like to refer you to an earlier column that I did on drifting insects. If you'll go to my recent column on FAOL, scroll down to "Previous Stream Doctor Columns" and then to the list under 2nd Quarter 2003, you'll find a column titled "Drifting Organisms?" If you'll open this, you'll find a column I wrote for Trout magazine on drift plus some other information on the subject. It pretty well sums up what we know about the three types of drift: active, behavioral, and catastrophic.

Second, I want to address your specific question on pool drift and the purpose and effect of drift. Then, in case others open the website you cite, I'd like to say a few words on the information (misinformation?) given in the article.

Drift can occur in pools, but it is mostly related to riffles and faster moving water. In fact, studies have shown a decrease in drifting insects as water enters a pool and the insects settle to the bottom. As to the purpose and effect of drift, at least the behavioral aspect results in a downstream dispersal of organisms. Competition for space and food resources can result in insects actively moving off of a site or being forced into the drift by aggressive competition from other insects. The effect results in dispersal of the insects throughout the stream, but as I mention in the FAOL column, a "colonization cycle" exists so that all insects don't end up in the ocean; mature egg-laying females fly upstream to deposit their eggs, thus resulting in an existing population in the headwaters. Part of the behavioral aspect of drift occurs as emergers leave the bottom to reach the surface; they can't help but being carried in the current as drifters at this time.

Now to the article on the website. If you read the column on drift on FAOL cited above, you'll find that drift can be active, that is, from insects actively leaving the bottom, but this is not for the purpose of obtaining food from the water column. It occurs when they leave the bottom, for one reason or another, and are swept up in the current; they aren't out for a swim! Of the various methods used by aquatic insects to feed (known as functional feeding groups; see my FAOL column, 2nd quarter 2004, "What do insects eat?"), none actively obtain food particles by picking them out of the water column. Thus, they do not feed on plankton (free floating organisms), as inferred on the website.

Knowledge on drifting insects is not relatively new. Published studies on this phenomenon have been available since 1972, and observations on drift of insects related to fish feeding were done as far back as the 1940s; a great deal is known about the subject. The website article also infers that drift only takes place twice a night and then for only minutes at a time; this is incorrect. Drift numbers in a stream are positively correlated with darkness; it increases and peaks soon after the sky darkens, remains relatively high during the night, and decreases again at dawn, with often a lesser peak just before dawn.

In fact, one researcher even found that during a cloudy night, drift numbers actually decreased momentarily for a short period when the moon appeared through the clouds for a brief period. There is some periodicity in certain drifting organisms, and I'm not disputing the fact that some particular organism exhibits the pattern the author of the website article described; what I'm saying is that the general pattern is one of high drift numbers during darkness and lower numbers during the day. Note that I'm not saying that there is no drift during the day; there is, or we'd be wasting a lot of time fishing drifting nymph patterns!

To further emphasize why drifting numbers are highest during darkness, observation by infrared light of stone surfaces during darkness revealed much higher numbers of insects on the upper surfaces during darkness than during the day. The obvious conclusion is that (1) the insects feel less susceptibility to predators during darkness, and (2) being on the upper surfaces of the stones, they are more likely to be swept into the water column; hence, higher numbers during darkness.

There's a good general discussion of the drift phenomenon in Tom Water's book Wildstream. He was one of the early scientists who studied drift.

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
Email: streamdoctor@aol.com

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 streamdoctor@aol.com.


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