The Stream Doctor

May 5th, 2003

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


Q. This topic resulted from a couple of sources and includes input from others than me. Al Campbell wrote a column ("Where to fish a hatch") in which he stated that stonefly nymphs are usually available to fish only as dislodged nymphs. In regards to this statement, Ask4saltydog wrote saying:

"I heard a couple of years ago that the nymphs actually ascend to the surface, float for a distance prior to descending to the streambed again and that the floating movement only occurred at night. Is this true?"

The following is from a column I published in Trout magazine a few years ago and summarizes much of what we know about drift and when it happens.

When fishing wet flies, nymphs, emergers, or any other sub-surface flies, anglers are usually imitating the immature form of aquatic insects. Collectively, these are the nymphs and larvae of the various aquatic insects and other true aquatic organisms that normally are attached to the substratum but also occur suspended in the water column. Biologists refer to this suspended community of organisms as "drift."

Scientists recognize three reasons or sources of these drifting organisms: normal drift that results from dislodgment of organisms by the current or from organisms actively entering the water column, behavioral drift that results from actions such as predator avoidance or competition for space, and catastrophic drift that occurs as a result of dislodgment by floods. Is drift purposeful? Most drifting insects probably don't enter the drift of their own volition - after all, they have no guarantee of finding a better site downstream and actually become more susceptible to predation by trout while suspended. But it does serve a purpose as a mechanism for recolonizing denuded areas resulting from such things as flood scouring, emergence, or human activities such as dredging, or rechannelization. It also allows organisms to compensate for overcrowding and competition for available food resources.

If you suspended a fine-meshed net in the current for a given time, say 20 minutes each hour over a 24 hour period, would you find the same numbers in each sample? Generally, the answer is "no". Intuition and some popular writing would predict that drift would increase as the waters warmed during the day, resulting in increased insect activity and enhanced opportunity to be dislodged. There are some isolated case for some species drifting more during the day, but the majority of studies of this phenomena show that drift is mainly a nighttime occurrence. In fact, one study not only showed increased numbers of drifting organisms during a cloudy night, it also showed that a brief emergence of the moon decreased the numbers of drifting insects. Why at night? During darkness, it is less likely that insects will be seen by predators, and increased activity and movement of the insects at this time enhances the likelihood of their being dislodged into the current. This increased activity during darkness has been documented by visual observance of aquatic insects using infra-red lighting.

So, how can fishermen use this information to their benefit? If you emphasize dry flies or emergers during a hatch, common sense tells you to concentrate on sub-surface patterns when drifting insects are most prevalent. This would be at night or under low-light conditions, or following spates or during high water conditions when the "catastrophic" component will increase. The latter doesn't need a lot of study to figure out, but good science was needed to document the night/day relationships.

Let me conclude with a question, which will lead to another interesting aspect of this community. If the immature forms of aquatic insects drift downstream and have been doing so for millennia, why are there any insects left in the headwater reaches? Why haven't they all ended up in the oceans by this time? Although it is true that the immature forms of insects are continually being moved downstream, with essentially no active movement upstream, it has also been found that the preponderance of adult insects flying upstream are gravid females seeking places to lay their eggs. Thus, a "colonization cycle" is produced where immatures in the water are moved downstream, but this is compensated for by the egg-laying females flying upstream. This cycle was first described by the Swedish stream ecologist Karl Müller in 1957, and has been verified throughout the world since then.

That's the end of the Trout column.

After receiving the original question from Ask4saltydog, I also had an e-mail from Matt concerning the subject of drift. A former professor of his added these pertinent facts from studies he had done on drift in Michigan and Pennsylvania streams, and I paraphrase from his e-mail: Mayflies and amphipods are important constituents of the drift. Stoneflies usually represent only a very small fraction of drift and tend to show only very weak periodicity (nocturnal maximum). Caddis are highly variable; some drift more at night and some more during the day. Drifting insects make no attempt to get to the water surface and drift densities are similar throughout the water column with only a slight bias to more in the water stratum immediately above the bottom. Drifting insects make no effort to float along for some fixed period; rather, they try to get back to the bottom quickly. Drift distances are very short, depending, of course, on water velocities.
~ 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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