The Requirements of Stream Trout

Fly Fusion editor Jim McLennan discusses the essential building blocks for the perfect trout stream.


by Jim McLennan

The basic building blocks of all life are water, oxygen, minerals and sunlight. Water may seem to be an obvious requirement, especially to a trout, but not just any water will do. It must be of the proper temperature, it must be free of toxic substances and it must carry oxygen and desirable minerals in quite specific volumes and proportions.

Oxygen must be present in the water at a concentration of at least four parts per million in order for trout to survive. However, water’s ability to carry oxygen is dependent on its temperature, and the warmer the water the less oxygen it can carry. Ironically, the warmer the water gets, the more oxygen trout need because they are cold-blooded creatures whose metabolic rate increases as the environment warms. In fact trout need about four times as much oxygen at 75° F as they do at 40° F, and this is a strong determining factor in where they can live. Trout need places where the dissolved oxygen level is consistently high or the water temperature is consistently cool or where there is some happy combination of both. Water temperature and oxygen content always go together, and when we read or hear about a fish-kill caused by water that is too warm, it is usually more accurately described as oxygen deprivation brought on by high water temperature.

Plant life in the stream also affects the dissolved oxygen content of the water. Through photosynthesis, green plants produce oxygen in the daytime, which is beneficial to the trout. However, the same plants use oxygen at night, and too many of them in the stream can rob the water of oxygen during warm summer nights. It can be a case of too much of a good thing.

The food chain in a trout stream goes something like this: Trout eat aquatic insects, aquatic insects eat zooplankton, zooplankton eat green algae, and green algae “eat” minerals. Without the right minerals there are no algae, no chain and no trout. There are many beneficial minerals, but the most important are calcium and carbon. Calcium reacts with oxygen and carbon dioxide to produce calcium bicarbonate, which is the mineral most easily “eaten” by the green algae. Water high in calcium bicarbonate is alkaline and carries a healthy batch of green algae, large aquatic plants, aquatic insects and ultimately trout.

The final critical ingredient is sunlight, which is used by green plants to help convert inorganic chemicals into organic matter fundamental to the food chain. Most of Alberta’s trout streams receive adequate sunlight, though occasionally suspended silt can inhibit sunlight’s penetration and limit the growth of algae on the stream bottom.

With the essential building blocks in place we have a place where trout can survive. But the addition of some other ingredients can make the difference between surviving and thriving. The first of these is stability, the absence of sudden or severe fluctuations in either temperature or volume of flow. Stable temperature keeps the trout and its food free of dissolved oxygen deficiencies and promotes growth of a diverse insect community. Stable flow allows fish to adopt a “routine” without having to move their place of residence every time the water level goes up or down suddenly.

Two other desirable ingredients in a trout stream are phosphorus and nitrogen. They act as fertilizers in the stream, just as they do on our lawns, stimulating plant production and promoting advanced growth throughout the food chain. Phosphorus and nitrogen can come from natural sources, like leaves and woody debris that fall into the water and decompose, or from artificial sources like cities and towns located along the waterway. Moderate amounts of these chemicals can be of great benefit to a trout stream, but excessive amounts can stimulate plant growth to the point where dissolved oxygen problems occur.

Other factors that enhance the productivity of a trout stream are an abundance of food and the presence of suitable habitat–neighbourhoods where the fish have safety from predators and shelter from heavy current. Trout also need access to suitable spawning water either in their home stream or in a tributary to it.

The perfect trout stream would be cool, stable and somewhat alkaline, with lots of green plants growing both in and around the water. It would have abundant insect life as well as plenty of hiding places for trout, and would have good spawning water nearby. That’s a long checklist and not many streams make it on all counts. One type that does is a spring creek, so called because its source is an underground spring. The spring releases water of constant volume and temperature throughout the entire year. The water is high in calcium, and its alkalinity stimulates plant growth. In addition, a spring creek is not part of the drainage route for mountain snowmelt, and consequently is unaffected by runoff. A spring creek’s groundwater source and constant temperature keeps large portions of it free of ice in winter. Spring creeks, also sometimes called limestone or chalk streams, are among the most productive and celebrated trout waters in the world. Sadly Alberta has only a very small number of spring creeks, the most noteworthy being the North Raven River, also called Stauffer Creek, northeast of Caroline.

Streams with more conventional snowmelt sources are called freestone streams, and most of Alberta’s trout waters fall into this category. They are usually not as rich in minerals nor as stable as spring creeks because of the nature of their sources. The best freestone streams, though, have adequate mineral content and undergo the inevitable seasonal changes in temperature and volume gradually rather than suddenly. This is often the case when the stream flows out of a lake or muskeg bog. The lake or swamp acts as a giant regulator, collecting snowmelt, adding minerals and releasing water to the outlet stream at a relatively constant rate. The Crowsnest River and some of the streams in the Peace River drainage are good examples.

Many people’s image of trout fishing comes from the clichéd calendar photograph or beer commercial showing an angler knee-deep in the headwaters of an icy creek with majestic snow-capped mountains rising straight out of the water behind him. He is usually in the process of netting a six or eight-pound rainbow. This looks wonderful, but I’m afraid it’s mostly Madison Avenue stuff. If I were a complete skeptic, I’d suggest that the photographer brought the huge trout with him as a prop.

Most streams like the ones in these photographs are not very productive and simply won’t have many big fish in them. The reasons are many. The growing season in the headwaters is short, and the climate is harsh. The gradient or slope of such streams is often steep so the water velocity is high and runoff scours the stream bed each spring. Even in places where the slope is more gentle, the water probably freezes right to the bottom in winter. Most significantly though, the water here is nothing more than melted ice. It is very pure, carrying few minerals and little phosphorous or nitrogen. The only sources of nutrients in the headwaters are the plants growing along the stream. The result is that the typical high mountain stream has very little algae growth and a very weak food chain.

In Mother Nature’s original plan, there was a fish capable of survival in these headwater areas. It is the bull trout, which has a voracious appetite and the ability to live long and grow large if man doesn’t interfere. But as we have seen, man has been interfering with Alberta’s bull trout for a century or so, and big ones are less common than they once were in their original headwater range.

So while the headwaters are not the best of trout neighborhoods, neither are the extreme lower ends of Alberta’s streams. A hundred miles from the mountains the rivers have slowed down and warmed up, and there are few riffles or rapids to inject oxygen into the water. They may now carry too many minerals because of the accumulated influence of cities, towns and irrigation. Perfect examples are the lower reaches of both the Bow and Oldman rivers in southeastern Alberta.

The middle sections of many of our streams, however, are a sweet compromise. As a stream moves from the mountains into the foothills, several significant things happen. Its gradient declines, which increases the amount of suitable trout habitat. It enters a friendlier environment at a lower elevation, which provides a longer growing season and reduces the likelihood of freezing to the bottom. A foothill creek flows through forest or farmland, and its streambed is composed of gravel, silt and soil instead of just rock as in its headwaters. Decaying leaves and plant material, animal waste and minerals wash in with the spring rains, providing nutrients to stimulate green plant growth. The stream also may flow through geological formations that leech calcium and other minerals into the water.

So trout fishers in search of Nirvana in Alberta find it in the streams of the western foothills, near towns like Pincher Creek or Sundre or Grande Cache, where aspens and willows mingle with evergreens to provide shade, cover and nutrients to the trout. Knowledgeable fly fishers are drawn to these gentle and aromatic valleys, knowing that finding trout begins with finding trout habitat.

Fly fishing is founded on the idea of trying to imitate the fish’s natural food with an artificial fly, so it’s helpful to know something about the creatures the fish eat. The next time you’re on your favourite trout stream, turn over some rocks and take a look. I do this when I teach a fly-fishing school, and everybody is always amazed that all those little wiggly things live under the rocks and in the weeds of the streambed. Everybody crowds around to look, and somebody always says, “I had no idea there was so much stuff living in the creek.” They’re amazed but they shouldn’t be. After all, the trout have to eat something or they wouldn’t be there, and they have to eat other living things. Trout feed on anything and everything in the stream that is alive and smaller than they are.

Trout food falls into two basic categories: aquatic and terrestrial. Aquatic creatures are those that have to spend at least part of their life in the water. They can be aquatic insects, other fish, or a miscellaneous group including leeches, freshwater crustaceans and aquatic worms. The most important of these are insects, and the most important trout-stream insects are mayflies, caddisflies and stoneflies. Terrestrial creatures are those that are supposed to live entirely on land. The ones we’re interested in (or more accurately the ones the fish are interested in) are those that live near the water but occasionally fall in or get blown in by a convenient breeze. This group always includes insects like grasshoppers, ants and beetles, but occasionally also includes things like clumsy mice.

Aquatic insects are so-named because they start their lives in the water. Mrs. Adult Bug lays her eggs on or in the water, and from the eggs come her immature children, called either nymphs or larvae, depending on which bug they are. The nymphs live in the water, feeding and growing until they are ready to become adults. This can take anywhere from a few months to a few years depending again on which bug.

At this point the three most important insects do things three different ways. Caddisflies seal themselves inside protective cases made of stream-bottom debris and undergo a transformation called pupation. The pupae then vacate the cases, swim to the surface and shed their skins and fly away as adults. In contrast, most mayfly nymphs simply swim to the surface of the water where they split their skins and step out as adults, which are called duns. The duns float on the water, waiting for their wings to dry before flying away. A short time later the duns molt again to become sexually mature spinners, which have long tails and clear, sparkly wings. Stonefly nymphs can’t swim, so they crawl to the edge of the stream and climb out on a rock or dead branch and make the change from nymph to adult on dry land. Once they’ve made it to adulthood the three bugs behave similarly again: They mate and the females lay eggs on the water and then die.

It’s important to know that different bugs live in different types of water. For example, stoneflies only live in rocky-bottomed, medium-to-fast velocity sections of freestone streams while some species of mayflies are only found in slow, silty areas. And within each group of insects are different species that come in assorted sizes and colours.

As a verb, the word hatch is used to describe the act of an aquatic insect changing from nymph or pupae to adult. It’s also called emergence. But the word has also taken on larger meaning as a noun to refer to the event of large numbers of a particular species of insect undergoing their emergence through the same time period, as in “the green-drake hatch occurs in the first half of July.” Insect hatches occur in the same order each year. So, just as the wolf-willows bloom in June but not in August, the Tricorythodes mayflies hatch in August but not in June. In fact the correlation between the hatching of insects and the blooming of wildflowers can help you with your fishing, and the idea is outlined nicely in Bob Scammell’s 1995 book, The Phenological Fly: A Guide to Meeting and Matching the Super Hatches of the West.

Every stream is a separate and distinct ecosystem with its own set of insects. So, while many streams share common insects, they may not have them in the same proportions. For instance, Prairie Creek and the North Raven River have both green and brown drake mayflies. On the North Raven both bugs are abundant and important, but on Prairie Creek the brown drake is the main event and the green is relatively minor. It’s beneficial to know which bugs are important on which streams, and you can learn this from other anglers, fly-fishing websites, videos, books like this one or by studying the streams yourself.

The insects we imitate with our flies have become symbols of the sport, and a stylized mayfly on a hat, shirt pocket or business card is like a secret badge identifying the person as a fly-fishing addict and linking him in spirit with other people similarly afflicted.

Fly fishers have always studied the bugs that trout eat, and some people wind up more interested in the bugs than the fish. That’s fine, but it’s not necessary to know the Latin name of the insect and the number of hairs on its kneecap in order to catch the fish that’s eating it. If you know what the bug looks like, where it lives and how it behaves, you can probably choose an imitation and fish it in a way that will fool the trout.

In fact, successful fly fishing can be boiled down to three easy steps. First, choose a fly the fish are likely to accept. Then put it where a fish will see it. Finally, make your fly behave like the natural food item it’s supposed to imitate. Simple, right? Well, kind of. There’s plenty there to keep us busy.

Above all, successful fly fishing requires patient, careful observation of the interactions between water, fish and insects. To demonstrate, I want to show you the good stuff–how a little knowledge can be a dangerous thing…to the trout. Let’s take an imaginary trip to a typical Alberta trout stream.

It’s late June, we’ve just arrived, and we need to decide what to do. We must take our clues from what we see, so we do our best to be observant. We look at the colour and level of the water, we note the wind and the sky, and we check the air and the surface of the water for insect activity. But it’s just a standard June morning, still a little cool, a few clouds, no breeze yet, no bugs on the water and, worst of all, no fish rising. Should we go home? I don’t think so.

We walk until we come to a long section of fast water broken by big rocks. There are still no fish showing, but we notice that every big rock along the bank is speckled with leftover shucks from some kind of insect. These are stonefly-nymph skins, and they could be just the clue we need. We know that for every empty shuck there is an adult stonefly around somewhere, and this tells us something about the fishing today and something else about the fishing a year from now. We know from the number of shucks on the rocks that a lot of adults are around, and we can presume that the fish will have seen and eaten some of them in recent days. They will have been conditioned to expect more today, so it makes sense to fish with an imitation of the adult stonefly.

When we get home, we make note of the date, water temperature and general weather conditions in our fishing diary, because the same hatch will take place at about the same time next year. We might want to be on the stream slightly earlier next year because the fishing will probably be even better then. Why? Because of what we know about stoneflies. We know the nymphs live in fast water and must crawl to the edge of the stream and climb out on dry land before hatching into adults. So, in the week or two prior to the hatch, every mature stonefly nymph in the stream is heading for the shore. Many are knocked loose from the rocks by the fast water, and because they can’t swim, they drift helplessly in the current. Guess who notices this? The trout see and eat more stonefly nymphs during the pre-emergence migration than at any other time of year, and the fishing with imitations of the nymph is often spectacular.

And that’s how fly fishing works. You develop a theory of what the fish are doing and try to confirm it by catching one on an imitation of their natural food. When you do catch a trout after this sort of observation and extrapolation, it’s especially satisfying because you know why you caught it. Of course it’s not always as straightforward as I’ve made it here, and there will be plenty of times when you feel more like Inspector Clouseau than Sherlock Holmes. But every once in a while the pieces all fall into place and you drift off to sleep that night knowing you’re a genius.