January 15th, 2001
The Premiere OnLine Magazine for the Fly Fishing Enthusiast.
Archive of Readers Casts
This is where our readers tell their stories . . .
Fish Can Really Smell
I have often wondered why a fish would follow my fly for
several feet, and come within an inch of eating it, but quickly
refuse it and turn away. Then one winter day when I was surfing
the Internet, I came across an article published by a marine
biologist at Louisiana State University. It was a study of the
olfactory [smell] and gustatory [taste] ability of catfish and
rainbow trout. Although there were some six-dollar words used
in the discussion of his experiments that were hard to
understand, I came to realize there might be an "odor factor" to
my flies that might explain the refusal mystery.
So I started searching for more and more information on the
chemoreception [smell & taste] ability of fish. I was primarily
interested in what the fisheries biologists had to say about the
subject, and believe me, there is a tremendous amount of
information on the Internet about these studies. If you are
interested, just go to your search engines and search for "fish
olfactory" or "fish gustatory" and you will have considerable
references to examine.
The primary purpose of this article is to make the fly fisherman
aware that fish, within their watery environment, are better at
smelling and tasting, than the bird dog is in the free air
environment. And this fact could possibly explain the refusals,
turn-around and spit-outs that all of us have observed over the
years. When you consider the variety of materials used to
construct a fly, plus the contamination you impart to the fly with
your own body acids, you never know what it will smell like to a
fish. The rest of this article is a summary of the research that has
been done, what are the compounds that offend the fish and
what can be done to eliminate the offensive odors.
Fish have two nostrils on each side of their head like this (::),
where water enters and exits. The olfactory [smell] system is
located between the entrance and exit nostrils on each side of its
head. There is no connection between the olfactory organs and
the throat, as there is in the human. Fish also have the ability to
taste, with taste buds on their lips, tongue, and throughout their
mouth. This is called the "gustatory" system in scientific lingo.
Some fish like catfish have barbels which are like whiskers that
It has also been determined that different species of fish
respond to different scents. This is because the scent
receptor sites are different in size, and they will only respond to
scent molecules that are the same size or smaller than the scent
receptor. In short, there is no one flavor that will stimulate all
fish species, also your fly may give off scent molecules that
are larger than the scent receptors in the fish you are
trying to catch, which is a real advantage. However, it is
reasonable to assume that larger fish have larger receptors and
have the ability to detect a wider variety of scent compounds.
WHAT IS "SCENT"?
What are these dissolved compounds that create "scent" that is
favorable to feeding behavior? Basically a scent is caused by
amino acids and things called pheromones, in molecular form.
Pheromones are chemicals that are used to communicate
between members of the same species during courtship and
mating or to signal danger when a predator is in the area. They
decompose rapidly, and are thought to have little to do with
Amino acids are the critical chemicals that affect feeding
behavior. Dissolved amino acids indicate the presence of living
or injured organisms to the fish, which may represent either food
or something foreign that should be avoided. Amino acids are
the molecular building blocks of proteins, which provide the
structure for all living things. Proteins are a necessary part of a
living cell, and next to water; protein makes up the greatest part
of body weight. There is some debate over the actual number of
amino acids because of the possibility of several combining to
make up others. Generally, there are 29 amino acids that have
been identified as performing different functions in the body.
Some are essential to life and some are not.
All living things, including man, continuously give off amino acids
in molecular form. The amount and type of chemicals given
off by a living organism depends upon its physical activity
as well as its emotional state. Catfish are sensitive to
excretions given off by other fish, such as urine and excrement
and other substances. A fish that is stressed will give off more
excrement than a "happy" fish. And a stressed fish may indicate
a "happy meal" to the catfish. Imagine the different chemicals
that a worm would excrete when impaled several times on your
fishhook. Now perhaps we can understand that there are
amino acids given off by emerging insects that fish can
readily detect. If the hatch is large enough, they may actually
flavor the water, which would cause a feeding frenzy.
HOW FAR ARE SCENTS DETECTABLE?
The distance from which fish can detect specific dissolved
chemicals can vary widely, depending on currents, water content
and the type of fish. Biologists have used very low levels of
amino acid stimuli, down to 1 part per 10 million in water, to
obtain a reliable electrical response from the olfactory nerves of
the fish under study. However these tests were conducted
under laboratory conditions. Bass are supposed to detect 1-
200th of a drop of attractant in 100 gallons of water. It has been
reported that Salmon, Sharks and Eels can detect certain
dissolved chemicals from as far as 500 miles. It has also been
determined that the memory of the home stream for a salmon is
not inherited, but is imprinted by the unique odors of the home
stream during the smoltification process when the young salmon
begin their downstream migration. This suggests that food odor
and other dissolved chemicals are learned and
remembered by the fish.
Just how acute the smell and taste ability is in a fish, is hard to
measure precisely. It has been demonstrated that a salmon
with its supernatural sense of smell can detect when a man
sticks his hand in the water 100 feet upstream! It has also
been said that the olfactory [smell] system in fish is several times
better than that of a good bird dog. The human nose has about
5 million olfactory cells and some dogs have over 200 million,
and a volume of four times that of our nose. That puts the fish
pretty high up on the smell-ability scale.
Okay, if the fish have such a terrific sense of smell, why are so
many caught on artificial flies, plugs, spoons, spinners and other
stuff. This can be explained primarily by the fact that the fish's
senses have been developed for the watery environment
in which it lives. Remember that the fish will respond if the
scent molecules are the same size or smaller than the scent
receptors. These receptors were developed to detect and taste
its basic food supply, such as insects, crustaceans, leeches, and
smaller fish of its own kind. The receptors were not developed
to sense fur, feathers, or other materials foreign to its watery
The second most significant factor is that vegetation, algae
and bacterial systems present in all fishing waters can
efficiently reduce the background levels of free amino
acids to exceptionally low levels. Therefore, the
detectable chemicals given off by an injured prey should
be fairly localized around its body. Perhaps this is the reason
why most fish species rely heavily on sight feeding. It also
explains why a fish may nose your fly and then refuse it,
expecting it to smell like something to eat, but when it doesn't, he
backs off. The only way to catch this fish with an artificial fly, is
to create a "knee jerk" reaction with your presentation, where
the fish has it in his mouth before he can taste it.
Many of the bait scents and so called fish attractants sold on the
market today do not really attract, but cover up the negative
scent combinations. In fact, many oil-based attractants would
be undetectable because their molecules are too large for the
receptor sites. Products such as Cossacks Bait Products
Shrimp, Herring, and Salmon Egg Oils; Riverside Lures Real
Craw; Smelly Jelly; Edge Products Hot Sauce; Fish Formula;
and Mikes Shrimp Oil are thought to be primarily oil based. I'm
sure there are many others.
Products that claim to be based on amino acids would be
Berkley's attractants along with Pharmacal's Baitmate Live and
Dr. Juices Elixirs.
Plant extracts are very effective in covering human odors.
Scents such as banana oil, garlic and anise are extracts from
plants. Some plant scents are Cossack Bait Products Anise and
Garlic Gel; Fish Formula Sparkle Scales; Mister Twister Banana
Oil; Mike's Glow Scent Jel and Anise Oil. Plant extracts do not
really attract, but provide an odor that the fish is not really
accustomed to smelling, yet is not offensive if used properly.
Fish "attractants" were developed primarily for the warm water
species like Bass. If you were fly fishing for catfish or carp, the
application of scents might be considered because they are
species that rely most heavily on smell and taste. But for the fish
species that rely primarily on sighting their source of food, my
recommendation is to forget formulated "fish attractants". There
are two reasons: the first is that the application of a formulated
scent cannot be accurately controlled. Too much scent is
worse than no scent at all, or even having a slightly negative
scent. The second reason is that if a scent gets too old, or too
hot, it may change into a toxic substance that does just the
opposite of what it was designed to do. Enough said for
Publishers Note: It is not legal in all states (or countries)
to use such scents. Always check local regulations.
PREVENTING OFFENSIVE ODORS ON THE FLY
The first thing an angler should do is to try and eliminate the
offensive odors that could be transferred to the fly. At the top of
the list should be insect repellents. If you use an insect
repellent on your hands, don't touch the fly. If you want to
sabotage your fishing buddy, just spray insect repellent in his fly
Next comes tobacco and nicotine. If you smoke or chew
tobacco when you are fishing or tying flies, take precautions so
that the smoke or tobacco does not come in contact with the fly.
Don't store flies or fly tying materials in empty tobacco tins or
Gasoline is pretty high on the list of transferable odors. Gas up
the night before your fishing trip or wash your hands thoroughly
before you handle your fly. Fly anglers who fish from boats with
outboard motors are likely to transfer this odor to the fly.
Humans give off a very offensive amino acid called L-Serene.
Of course this scent is transferred to the fly you tie or handle.
(And whatever you do, don't spit on the fly for good luck.)
Never wet your fingers with saliva to apply dubbing. Although
you can't eliminate it totally, washing your hands with Lava or
Ivory bar soap before tying flies or handling them will help
immensely. Wetting your hands in the stream or lake before
tying on a fly will help.
Another major offensive odor to fish is sun tan lotion, and just
about any other hand lotion or soap that is designed to be
fragrant and lubricating. Dishwashing detergents are known
to be offensive to fish. So don't wash your hands with liquid
soaps before tying flies.
The actual fly itself can contain a host of scent combinations.
Although there has never been a study to my knowledge of the
resident chemicals and scents built into a fly, you only have to
imagine the possibilities. Since there is no practical way to inject
an attractive flavor (an amino acid that duplicates the living
organism) into the fly, I prefer a "no-odor" fly, and hope my
presentation is good enough to get it into the fish's mouth before
he tastes it.
One of the best odor cleansing operations you can give your fly
is to use the vegetation, moss, mud and weeds that are present
on the waters you are fishing. Don't cuss the next time you
retrieve a load of moss or weeds on your fly, you have just
made your fly more acceptable to the fish. ~ Phil Garberich
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