Hatches Made Easy:
Great Smoky Mountains - Aquatic Insect Update:
Winter is the best time to take samples of the aquatic insects in the park. Since
most of the hatches haven't occurred, most of the aquatic insect larvae are
present. If you take samples in June, for example, many of the insects (those
that hatched) would either be eggs or undeveloped larvae.
We have a permit from the National Park Service to do this. We cannot keep or
collect the insects, and have no reason to do so, but we can catch and
photograph them. We have taken periodic samples of the larvae hundreds of
times from several different streams in the park. We have the proper equipment
to do this.
Last year we noted that the average quantities of some species of mayflies,
stoneflies and caddisflies in a given area of bottom changed from the year
before. This year (or late last year 2007) we found that the quantities changed
even more drastically.
We cannot find any studies that have been done by entomologist regarding
this. Although our finding are not scientific findings they are a very good
indication that the population of certain species of mayflies, especially the
clingers, is down a lot. The number of Quill Gordon and March Brown nymphs,
for example, is down drastically from the same thirty-five test areas we sampled
the year before. On the other hand, certain species of caddisflies that thrive in
slightly warmer water have exploded in some areas.
Now I am aware that these populations vary from year to year, but I feel very
confident in saying the drought conditions have changed the populations
of some species of aquatic insects considerably.
This past summer and fall months during the time the drought was at its worst,
we took DO (dissolved oxygen) readings using an electronic DO meter along
with the PH and water temperature from several different streams on several
different dates. In many cases, the amount of dissolved oxygen was very low,
probably too low for some of the aquatic insects that require a lot of oxygen.
This is mainly the clinger nymphs consisting of both mayfly and stonefly species.
It is my understanding that trout require about 3 ppm or parts per million DO to
survive. At 75 degrees F., this is about the maximum amount of DO water can
hold. At this temperature, if there is no turbulence to assist in adding oxygen to
the water, the trout will probably die from suffocation. When the water is above
68 degrees F. the trout's metabolism is racing. Unless the water is very
turbulent, the oxygen level will drop drastically. When this happens the trout are
burning the oxygen faster than it is available. They will normally try to adjust to
this by trying to find more oxygenated water. Finally, they will simply not move
around very much.
In many cases we found the dissolved oxygen content of the water was far
too low for the trout, especially the rainbows. Not only was the water warm,
the normal plunges and runs didn't exist. Where riffles normally exist (which
would help add oxygen to the water) the water was smooth and the flows were
slow. Long stretches of major streams just didn't get the oxygen they needed for
the insects or trout due to the low, warm water conditions.
The temperature of water is inversely proportional to the amount of dissolved
oxygen. To make this simple, water in the seventy degree F. range holds a heck
of a lot less oxygen than water in the sixty degree range. It does not change
proportionally. It changes like a jet changes speed when it hits the runway.
The bottom line to this is that we expect the hatches of clinger mayflies and
stoneflies to be far less in terms of quantities of insects than they usually are.
The numbers of clinger and swimmer nymphs are also low, but not as low as the
clingers. After all, about half of the bottom area of the normal stream
substrate was dry terrain for a long time this past summer. Since most of the
parks mayflies and stoneflies are clinger nymphs, I expect this will have an
impact on the size and duration of the hatches.
Now I'm sure there will be those that will go out on a stream this year and see a
huge hatch of mayflies and swear that there are as many of them as there ever
was. If conditions are right, hatches can be condensed into a short time period
and make it appear that the hatch is huge. Other times, the hatches may start,
stop, start, stop and dwindle along over a long period of time and appear the be
very low in intensity. It is almost impossible to measure the number of insects
that are hatching by observing the mayfly duns or adult stonefly or caddisflies.
The bottom nymphs and larvae samples don't lie. It is not difficult to determine
the species and quantities of them that are in a stream. That is not exactly
Caddisflies of some species, such as the Brachycentrus, are much lower than
last year. Like the clinger nymphs they too need cold water with plenty of oxygen.
In Yellowstone National Park, the population of aquatic insects in the Firehole
and Madison River has drastically changed in the last three to five years. That is
because they have had drought conditions (the worst this past year) and low
snow packs in recent years. Studies have been done there to confirm this but to
our knowledge, none have been done in the Smokies. Many studies pertaining
to insects are ongoing in the park but those that have to do with the things us
anglers are interested in, don't exist. I certainly understand why. Many of the
studies that have been conducted were very important in many other respects.
It is my understanding from a few samples the park fisheries people have taken
that the drought lowered the population of trout, especially the rainbows. That is
probably a good thing. Maybe that is how this all will work out. One thing for sure
- there is certainly not as much food available for the trout to eat as there
normally would be.
Coming Up Next:
Copyright 2008 James Marsh