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?”
A.
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 Méller 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.
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.
Originally published February 2nd, 2004 on Fly Anglers Online by Robert Younghanz.