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
"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
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