January 15th, 2001

The Premiere OnLine Magazine for the Fly Fishing Enthusiast.
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Fish Can Really Smell

Phil Garberich

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.

THE BASICS

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 can taste.

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 feeding behavior.

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.

MITIGATING FACTORS

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 environment.

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.

FISH ATTRACTANTS

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 "attractants".
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 box.

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 cigar boxes.

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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