How Many Trout Are in That Stream?

The number and size of trout a stream can support are always limited by something, but almost never by fishing pressure or other predation. Populations are usually limited by the physical features of the stream, and you can make predictions about how many trout a stream holds by an estimation of its richness. Infertile streams have little migration, stunted adults, and many juveniles, Rich streams, on the other hand, are space-limited. Trout can get enough food anywhere in the stream, and the total number of trout is limited by the number of available places to hold and feed without wasting an inordinate amount of energy. A rich stream with a bottom covered with rubble of different-sized rocks offers lots of nooks and crannies to break the force of the current, and it can hold many more trout than a stream of equal richness with a sand or gravel bottom. A spring creek with many weedbeds offers protection from the current and places to hide when danger threatens, and it can hold more trout than an equally rich stream that has been widened, shallowed, and trampled by cattle.

Can you spot all the trout in this photo? A spring creek offers enough food to keep all of these fish happy, even though they're in close proximity.

A food-limited stream hosts trout of many different sizes, with frequent interactions among individuals (in competition for space) and net migration downstream. This migration, usually of the largest individuals in a population, can help you find some interesting fishing on today's crowded waters. The lower reaches of many of our richest and most famous trout streams offer fishing for big trout in water usually thought to be the home of bass, northern pike, walleyes, and even carp. These lower-river trout seldom respond to hatches, if indeed there are any in such warm-water habitats, so prospecting techniques will help you find them. I've explored the lower Beaverkill and Delaware in the Catskills, well out of the famous trout water, and the lower Battenkill, and I've found surprisingly good fishing for large brown trout. Friends tell me about equally good fishing well out of the supposed trout range on the Madison, Bighorn, and Missouri.

Spawning conditions are poor in these places, and most of the trout ascend to the upper river to spawn, or they use a tributary stream. As you might suspect, water temperature is the main limiting factor for trout populations in these places. You can confine your search to the mouths of tributary streams, especially in the summer, and that is a relief because the lower reaches of these rivers are often huge. On one mammoth river in the Northeast (I would be risking the attention of a hit man if I used the name), there is a tight­lipped group of local fishermen who fish a deep, wide stretch of water a hundred miles below what is considered by the local chamber of commerce to be trout water. At the mouth of each cold­water tributary stream is a whirlpool, and just before dark these fishermen launch float tubes into the whirlpools and slowly revolve into the sunset. In a stretch of water known for walleyes and smallmouth bass, they catch rainbows that average about twenty inches long.

Why Are Some Streams Richer than Others?

Aquatic crustaceans like this scud, show here with an imitation, indicate rich water.

Now that you know some advantages to learning to gauge richness, let's explore the reasons for these differences in productivity. Calcium compounds, found in many sedimentary and metamorphic rocks, counteract acidity. The amount of biomass in a stream, particularly the pounds per acre of trout flesh, is directly related to the pH. The more bicarbonate in solution, the more acid is neutralized, and the higher the pH. The higher the pH, the more productive the trout stream. Calcium in solution is also suspected to benefit the physiology of trout directly. Apparently it helps the fish fight off toxins in the water. Plants can also pull a molecule of carbon dioxide directly off calcium bicarbonate in solution, so streams with high calcium content support more plant life. More plants mean more insects. And fatter trout.

The amount of calcium in a stream also determines the supply of one type of organism that is extremely valuable as a trout food â?? crustaceans. The outer shell of these animals is made from a compound high in calcium, and because they absorb it directly from the water, the abundance of crustaceans in a stream is directly related to the. concentration of calcium bicarbonate. Crayfish, sow bugs, and amphipods (or scuds, as fishermen call them) are a high­energy source of food year-round. These animals do not hatch out of a river as aquatic insects do, so toward the season's end and throughout the winter, when mature insects have flown away and mated and their offspring are too small to be of much use, full-sized, adult crustaceans are available. Crustaceans are easy to capture and high in protein and fat. Wherever they are found in great numbers, you will find lots of corpulent trout. Nymph fishing is superb in streams with large populations of crustaceans, to the point where trout often ignore heavy mayfly hatches because crustaceans give them a source of high energy without the costly risk of surface feeding. The chapter on nymphs will give you some ideas on how to take advantage of this opportunity.

A Brief Field Guide to Rich and Poor Trout Streams

Two minerals form the bulk of freshwater buffering systems: calcite and dolomite. Calcite is pure calcium carbonate, and dolomite is mostly calcium magnesium carbonate with various impurities. Limestone is the most common source of these minerals, and the most productive trout streams in the world flow through limestone. You can spot limestone bedrock by its sedimentary layers and its brown or yellowish color. Because limestone occurs in flat layers, rocks along the banks of a river with limestone are flat plates, as opposed to the rounded igneous or metamorphic rocks of less fertile streams. Sandstone and shale, sedimentary rocks that don't contribute much to the fertility of a stream, can be distinguished instantly from limestone or dolomite with a couple of drops of vinegar or dilute hydrochloric acid. Rocks that contain calcite or dolomite effervesce or fizz when you put the weak acid on them. Did you ever think you could predict how to fish a trout stream by carrying a vial of vinegar with you?

Water that looks like this will be calcium-rich, and full of aquatic insects and crustaceans.

Gypsum and marble also buffer trout streams and make them richer because they contain calcium carbonate. You can tell them from other whitish rocks, such as quartz, because they lack the large, crystalline grains you see in rocks that contain quartz. They also usually have a crumbly look, which comes from their solubility in the weak acid of rainwater. Marble is metamorphosed limestone or dolomite, and because of the heat and pressure it has undergone, it is not as soluble as limestone (sometimes you have to pulverize it before it will fizz with weak acid), but marble still offers strong buffering properties. The Battenkill flows through a valley flanked by insoluble granite and gneiss to the east and marble bedrock to the west. Its pH fluctuates from around 5 below tributaries or springs entering from the east to more than 7 downstream of tributaries entering from the west. Trout in the brooks on the eastern slope will take a big dry fly all day long, regardless of the insects hatching or the time of day, but if you hop over to the other side of the valley, the trout often ignore a blind-fished dry fly. You have to use smaller flies that look more like the insects that are hatching during the current week, and the trout seem to have periods of lockjaw when no fly will work. The trout in the western tribs are also bigger and fatter.

Rocks composed mainly of silica contribute nothing to the productivity of a trout stream because they release no carbonates into the water. Silica rocks in streams can be recognized by their smooth, rounded shapes and crystalline structure. The ones you commonly see making up the beds of unproductive trout streams are gneiss, sandstone, quartzite, and various forms of granite.

These limestone ledges along the Madison in Montana betray its richness at a glance.

Seldom do I predict the richness of a trout stream solely by staring at the rocks. It's easier and more accurate to eyeball other clues in and around a stream and use the geology as one piece of the puzzle. For example, the color of the water can often be a dead giveaway to its richness. The tea-colored water so common in the north country indicates an infertile stream, where trout will be small, slow-growing, and eager to take almost any fly pattern. In the limestone belt of Pennsylvania many of the streams have a gray or white tint due to undissolved calcium carbonate, and the trout are well-fed, pickier about what nymph they take, and less inclined to come to the surface for a blind-fished dry fly. Water with no apparent color is not much of a help â?? it can indicate either a stream where all the brownish humic acid has been neutralized by carbonates, or, as in many high-altitude streams in the Rocky Mountains, water that has few dissolved minerals of any kind. Crystal-clear water can indicate purity, but absolutely pure water is less productive than water that contains some dissolved nutrients.

Continue Reading "Rich and Poor Trout Streams"   1  2  3  4