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 40 feet deep rings the outer edges towards shore. The lake has abundant shallow feeding areas with nearby deeper water where its rainbow trout can escape predators and access 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 20 feet deep. This means energy from the sun penetrates to the lake bottom and allows photosynthesis to produce lush emergent green plant growth. This organic matter at the bottom of the lake provides the habitat for many trout food sources as well as providing cover for juvenile trout. Each winter, the decomposing vegetation provides 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 productivity. 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 summers with elevated air temperatures.
The drop-off is the transition zone between the shoal and deep-water areas of a lake. The slope of the drop-off can vary from 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. The most productive stillwaters are seldom deep, usually with maximum depths of less than 50 feet. 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 30 to 60 feet deep. At other times of the year trout seek out the deep-water zone in search of diving water boatman and backswimmers.