Locating Trout in Lakes

By Brian Chan

Fishing a large marl shoal, ideal scud habitat.

Determining where trout live in lakes and, in particular, where they feed is critical to fly-fishing success. It is definitely worth the time to learn some basic facts about lake morphometry, biology and water chemistry. This knowledge relates directly to understanding the life-cycles of important aquatic food sources, including significant insect emergences. So let’s take a more detailed look at the stillwater trout environment, where these fish live, their preferred feeding habitats, and the impacts of changing water chemistry.

Structure of Lakes

Big OK or Island Lake, located southwest of the city of Kamloops, British Columbia, is a near perfectly designed trout lake. A large table-top shaped shallow water area covers much of the middle portion of the lake, while a deep-water trough that reaches almost 12 meters in depth rings the outer edges towards shore. The lake has abundant shallow feeding areas with nearby deeper water for the rainbow trout to escape predators as well as provide refuge from warm summer water temperatures. All productive trout lakes, regardless of geographic location, have one physical feature in common—they typically have a large littoral or shoal zone, which is defined as water less than six meters deep. This means energy from the sun penetrates to the lake bottom and allows photosynthesis to produce lush submerged and emergent green plant growth. This organic matter provides the habitat for many trout food sources as well as providing cover for juvenile trout. Each winter, the decomposing vegetation provides the nutrients for the next season of plant growth.

The typical small lake is shaped like a dinner plate with shallow edges sloping off into a deeper center area. The amount of shallow shoal area in relation to the deep water area gives a strong indication of relative productiveness. However there is a fine balance between the amount of shallow water versus deep water area and survival of trout in lakes that endure months of ice cover and/or experience long hot, dry summer air temperatures.

The drop-off is the transition zone between the shoal and deep-water areas of the lake. The slope of the drop-off can vary from very gradual to an abrupt angle almost approaching 90 degrees. The more gentle slopes offer more diverse fish and fish food habitat and fishing area for that matter. The drop-off plays a crucial role during warm water periods as the water temperatures drop significantly over a short vertical distance and provide a refuge for trout that seek cooler more oxygenated water.

Beyond the drop-off is the limnetic or deepwater zone of the lake. The most productive stillwaters are seldom deep, usually with maximum depths of less than 20 meters. Some of the best western trout fisheries are less than eight meters in maximum depth. The most prominent trout food source found in the deep-water zone is zooplankton. Common zooplankton are the crustaceans Daphnia, Cyclops and Bosmina, which are all quite visible to the naked eye. Zooplankton gather in dense horizontal layers or bands in the water column and fish just swim through the masses for a nutritious and easily captured meal. This deeper water can also be a large refuge area for trout during the hot summer months. The abundant supply of tiny zooplankton in deep, cold water creates the situation known to anglers as the summer doldrums, where the fish are deep and gorging on food items that are difficult to imitate with a fly. The deepwater zone is typically devoid of rooted plant life, which limits the diversity of aquatic invertebrate food sources. The most abundant insects found living in deep water are chironomids, as the rich organic bottom or benthic zone provides ideal larval habitat. It is not uncommon to have chironomid pupal emergences occurring in water from 10 to 20 meters in depth. At other times of the year trout seek out the deep-water zone in search of diving water boatman and backswimmers that are in the midst of swarming and mating flights.

The Importance of Aquatic Vegetation

Aquatic vegetation plays an important role in small lake ecosystems. Submergent species provide prime habitat for scuds, mayfly, damselfly and dragonfly nymphs, caddis larvae, snails, water boatmen, backswimmers and leeches. Common submergent plants include potamogeton (Pondweed, Cabbage Grass), milfoil and ceratophylum (Coontail, Hornwort). These plants are rooted and grow up from the bottom of the shoal and drop-off zones. A quick sweep through these plants with a small aquarium net or kitchen strainer reveals some or all of the above mentioned invertebrates. The most important of these rooted green plants is chara, or Stonewort, a green algae that forms a skeleton of calcium carbonate. Chara is easily identified by its crunchy, brittle texture and strong musty odor. It often forms a thick covering over the bottom of the shoal in lakes that are rich in calcium. Chara covered shoals offer ideal shrimp or scud habitat and its presence identifies a very nutrient-rich waterbody.

Emergent vegetation, such as cattails, hard-stemmed bulrushes and lily pads offer cover to fish swimming in shallow water and also provides the emergence habitat for damselflies. Mature nymphs swim up to the underwater stems of these plants and then crawl up and out of the water. Their nymphal shucks dry and split open as the adult form emergences and rests on the plant stems or leaves. Growing around these plant stems are other submerged vegetation, which in combination make for ideal foraging habitat for both juvenile and adult trout.

One can start to see why so much time is spent chasing trout in shallow water. During the spring-to early summer period this area is alive with aquatic insect activity and emergences along with the ongoing presence of other staple foods, such as scuds and leeches. Trout return to the shoal zones again in the fall when water temperatures have cooled down. They now search of the juvenile or younger instars of the same aquatic insects and other invertebrates that made up their spring diet.

Seasonal Changes in Water Chemistry

Spring Turnover

Trout are cold-blooded and their metabolism is directly related to water temperature and associated dissolved oxygen levels. During the open-water period there are three limnological events that occur. These determine where in the lake the trout can live. At ice-off the entire water column is cold. But on close examination we find the bottom layers of water are actually a few degrees warmer than the surface waters. The benthic water is warmer due to heat released during the decomposition process of plant matter that has occurred through the winter months. Of course some of these lakes are covered with ice and snow for up to six months a year. Two major mixing events occur each year in almost all small trout lakes in the northern hemisphere. Spring turnover typically occurs within a few days to a couple of weeks after ice-off. Warmer air temperatures slowly warm the cooler upper layers of water until the temperature of the entire water column is similar. Water is most dense at 4 degrees C (39 degrees F) and at that point mixes easily with a little assistance from the wind. A lake in turnover becomes quite turbid or murky as detritus is swept off the lake bottom and mixed throughout the water column. Spring turnover is an important event as it ensures the entire water column is saturated with oxygen. However, fishing during a turnover event is generally not very good as the overall water chemistry is poor and the fish generally become dour. It generally takes three-to five days to complete the turnover and for the water to clear. The completion of turnover signals the start of the spring fishing season as continually warming water initiates the parade of aquatic insect emergences.

Summer Stratification

Long, hot summers are a key ingredient in producing a quality stillwater trout fishery. Photosynthetic energy drives the aquatic ecosystem by stimulating phytoplankton and zooplankton growth, aquatic plant production and development and maturation of insect populations. Under optimum conditions this all translates into significant fish growth. However, trout are cold-blooded and their metabolisms are affected by water temperature and related changes to water chemistry.

By early summer, the upper layers of water in many small lakes begin to warm and become less dense than the colder, deeper water. In other words, the energy from the sun penetrates to a certain depth and beyond that the water stays much cooler. A temperature gradient known as a thermocline develops where the warmer upper layers, known as the epilimnion, meet the colder, deeper water or hypolimnion. Thermoclines are invisible and typically establish between five and eight meters deep and they are a barrier to mixing. Therefore, summer winds continually warm and circulate the water of the epilimnion while the deeper hypolimnetic water below remains unmixed. Ongoing bacterial decomposition occurring within the benthic or bottom areas of the deepwater zone results in decreasing levels of oxygen below the thermocline. In many of the most productive small trout lakes there may not be enough oxygen below the thermocline to support fish life. In very hot weather trout leave the warm shallow shoal areas and sit just above the thermocline where the water is colder and well oxygenated. Trout suspended along the thermocline show up well on a properly calibrated fish-finder. Trout often come back onto the shoals to feed at night when water temperatures are cooler and oxygen levels hit the appropriate concentrations. That is why we experience some great late-night summer fishing in water often less than three meters in deep.

Fall Turnover

By early fall the temperature differences between the upper and lower layers of water is reduced and the thermocline breaks down. By mid-fall, air temperatures will have cooled upper water layers significantly and once the entire water column is isothermal, or of similar temperature, and as close to 4 degrees C as possible, the entire lake will mix with the aid of a good wind. As in the spring, the water becomes murky for a few days and then clears. Fall turnover is critical to the winter survival of trout, as this late-season mixing action ensures the entire lake is well oxygenated prior to freeze-up.

Although short in duration, late fall fishing provides some of the most exciting action of the year. Trout feed very aggressively before the onset of ice cover. In a nutshell, it is shallow water fishing for some of the biggest trout of the season. The fish are looking for just about any food item that moves, but imitations of leeches, scuds, damselfly and dragonfly nymphs and chironomid larvae top the grocery list.

Matching Fly Lines to Habitat

Having a good understanding of where trout live and feed in a lake makes it much easier to determine which fly lines should be in the stillwater tackle bag. A floating line can be used extensively as so much feeding occurs in water less than eight meters deep. Floating lines with varying leader lengths can be fished with or without strike indicators. The choice of setup is dependent on the food source being imitated and specific depth zones being targeted. An intermediate sinking or slow sinking line is another must-have line for the serious lake fly fisher. These lines can be fished in shallow water. And, by varying the wait time before initiating a retrieve, these coverage of any part of the depth zone being fished. A full-sinking line in fast-to super fast sinking (Type 3 to Type 7) allows coverage of the steeper drop-offs and also any fishing situation encountered in the deepwater zone. Another excellent lake-fishing line is an emerger tip, meaning a line with short, five-foot-long intermediate sinking tips. These mini sink-tips are great for fishing chironomid pupae, mayfly nymphs, damselfly nymphs and caddis pupal emergences.

A good background in basic lake structure and the biological processes that occurs each year in your favorite fishing lake makes it a lot easier to learn about the various aquatic food sources that trout eat, including their life-cycles. This arms you with the knowledge you need to present the correct flies to trout, at the proper depths, in any season.