Anatomy of a Trout Lake

Brian "Mr Chan" coming to you today! You are going to very pleased at what he has to say about the Anatomy of a Trout lake. Sit back, relax, and enjoy the knowledge he will be providing you.

I have to admit that attending university was not all that much fun for me. The exceptions were a 3 hour break every Wednesday for intramural hockey and taking course that related to improving anything about my understanding of fish and improving my fishing skills. Courses that stood out for me included freshwater ecology, aquatic entomology, limnology (study of lakes) and the biology of fishes. I still regularly refer to the now old textbooks from these courses. There is no question that understanding the overall ecology of a trout lake will improve your fishing success. Relating where trout live in relationship to food sources, habitat availability and suitability will definitely help catch you more fish! So let’s take a crash course in the structure and ecology of trout lakes as it relates to increasing quality time on the water.

In British Columbia, not all lakes are created equal. Those water bodies located in the interior regions or east of the coast mountain range are typically much more nutrient rich or more productive than coastal lakes. A variety of factors contribute to these differences but the most significant reasons are how these lakes were created coupled with geographical and climatic conditions. Many of the most well-known interior trout lakes were created during the last glaciation. Receding glaciers scoured out literally thousands of relatively shallow depressions in the landscape. These filled with water over time and many have become the fishing lakes we know today. The interior plateau region extends from north of Fort St. John and travels south through the Cariboo and Thompson/Nicola areas and continues down to cover the Okanagan and Kootenays.

Interior trout lakes are able to grow fish because of several key factors. Climate conditions feature long, hot summers with abundant sunlight. This is coupled with a more nutrient rich land base that sees higher levels of key elements such as calcium, phosphorus, nitrogen and magnesium. Add water to the picture and you have the basis for a fertile lake environment for all things aquatic. The interior regions of the province receive much less total precipitation than that of the coastal regions which reflects directly on the amount of runoff entering the typical trout lake. Many of these fisheries have only seasonal inlet and outlet creeks or are totally landlocked. Therefore the water in these lakes has a long residence time, or in other words, nutrients stay in the water body. 

 Small coastal trout lakes, in general, do not support high enough concentrations of the key nutrients to grow fish as big as those found in their interior counterparts. This is due in part to prevailing geological conditions and increased amounts of total precipitation which result in high flushing rates. Invertebrate life in coastal lakes is generally less diverse and abundant than those found in the interior regions.

Structure of Trout Lakes

The typical small trout lake is composed of 3 distinct habitat zones made up by the shoal, drop-off and deep water areas. The shoal starts at the edge of the lake and extends out, gradually getting deeper until reaching the edge of the drop-off. Here the water quickly deepens to the deep water or limnetic area of the lake. Some lakes have extensive shoal areas and only small deep water zones while others have minimal shoals and extensive deep water habitat. The most productive lakes for trout are those that have at least an equal or slightly higher percentage of shoal versus deep water habitat. The shoal which is also referred to as the littoral zone is the grocery store of the lake as well as offering prime living habitat for trout.

The Shoal

Biologically the shoal is described as the water in the lake that is less than 6 meters or 20 feet in depth. This is where the energy from the sun can reach to the bottom of the lake and allow green plant life to flourish. The lush vegetation growing on and up off the bottom of the shoal zone supports a wide variety of aquatic invertebrates that form the basis of the diet of trout. Reach over the side of your boat or walk along the edges of the lakeshore and pull up a clump of vegetation and watch what starts crawling out of it. Freshwater shrimp or scuds, damselfly nymphs, mayfly nymphs, caddis larvae, dragonfly nymphs , water boatman and backswimmers are all commonly found hiding in the vegetation. It stands to reason that the more shoal area and accompanying plant growth in the lake increases the overall productive capacity to grow trout. Common aquatic plants found in small trout lakes includes pondweed, coontail, Hard-stemmed Bulrush, cattails, lily pads and native milfoil. Many of the most productive trout lakes will also have dense mats of a green algae called Chara or stonewort covering large areas of the shoal bottom. Chara flourishes in calcium-rich waters and the green stems and whorled branches are often encrusted in mineral deposits making it very brittle to the touch. It is easily recognized by its very strong musty odour. Chara is prime real-estate for shrimp, chironomid larvae, dragonfly nymphs and damselfly nymphs. Chara laden shoals are often interspersed with a marl lake bottom. A marl bottom looks like yellowish-white sand but it is calcium and other mineral deposits that form a shallow layer over an otherwise rich organic muck bottom. It is also superb scud, leech and dragonfly nymph habitat.   Trout can spend a lot of time cruising over marl and Chara lined shoals looking for their next meal. In clear water lakes you can watch these fish tip their heads into the marl or Chara as they suck out a scud or chironomid larvae. Much of this type of feeding is done in water less than 10 feet deep and often in less water than that. 

Other aquatic plants such as pondweed, milfoil and coontail grow in dense mats forming almost forests of vegetation that cover parts of the shoal zone. Trout will patrol the outer edges of these weed lines or forage in amongst the plant stems picking off mayfly, damselfly and dragonfly nymphs as well as scuds and caddis larvae. Bulrush and cattail plants often line the shallow edges of the lake as well as extending out into the main shoal zone. These are emergent plants whose stems and seed pods protrude above the surface of the lake. Both plants species play an important role in the completion of the damselfly life cycle. Mature nymphs swim en masse to these stands of plants then climb up out of the water on the plant stems. Once out of the water the nymphal shuck hardens and splits allowing the adult form to emerge and eventually fly off.

The Drop-off

The drop-off is the transition between the shoal and the deep-water zones of the lake. The angle or slope of the drop-off dictates whether it is a small or large piece of trout habitat. Gradually sloping drop-offs offer more feeding areas for fish and therefore more fishing opportunities. The shallower parts of a drop-off are still under the influence of photosynthesis so plant life will extend down for at least some distance. Drop-offs are also key refuge areas for trout. While intense feeding can occur on the shoal, the shallow water does not offer protection from predators such as loons and ospreys. The gradually deepening water of the drop-off provides the cover needed while the fish rest and digest food. As the water deepens it also cools and the drop-off becomes an extremely important refuge area for trout during the warmest months of the year. Here the fish find well oxygenated water plus food sources that they are familiar with. In some lakes the slope of the drop-off is very steep, almost vertical and thus does not provide as much useable habitat as lower gradient ones.

The Deep Water Zone

The deeper parts of the lake generally do not support the diversity of aquatic invertebrate food sources as compared to those found in the shallower areas. However, the deep water is an important refuge for trout during specific times of the year.  In the typical interior trout lake the deeper water is suitable habitat during the period from the completion of spring turnover to the initiation of summer stratification and again from the completion of fall turnover to freeze-up. These are times when oxygen levels are high throughout all depths of the water column. The deep water zone supports dense populations of zooplankton such as Daphnia, Bosmina and Cyclops. These are the tiny red or green crustaceans you see suspended in the water. Often, they assemble in large masses or clouds in the water, all visible to the naked eye. Zooplankton are sensitive to light so during daylight hours they migrate to the deeper parts of the water and as nightfall arrives they move back up higher in the water column. During the summer months trout will feed extensively on zooplankton, often gorging on them in deeper water or just above the thermocline that forms on many small lakes. Thermoclines mark the extent of heating from the energy of the sun. Thermoclines are invisible barriers that separate the warmer upper layers of water from the colder, deeper water. The depth at which they get established is related to water clarity. Thermoclines generally develop at depths between 15 to 25 feet. The water below the thermocline does not mix and in the most productive trout lakes this deeper water can become quite low in oxygen.  In the most extreme situations trout cannot survive below the thermocline due to anoxic conditions.

Deep water chironomid emergences occur in many trout lakes and in some of the larger lakes there can be some great pupal fishing in very deep water. This is the case in lakes such as Whitetail in the East Kootenay, Sheridan in the Cariboo and White in the Thompson/Nicola region. Anglers will be fishing chironomid pupal patterns in water as deep as 75 feet. The trout are down at the bottom gorging on emerging pupa. Obviously there is ample oxygen for the trout at these depths.

In the mid to late fall period the deep water zone again becomes an important feeding zone for trout. Water boatman and backswimmers travel from one water body to another during swarming or mating activities. These are air breathing insects that are excellent fliers as well competent swimmers. They fly in large numbers from lake to lake, diving into the water to complete egg laying duties. Trout zero in on the insects as they hit the water and then as the bugs swim quickly down through the water column. This annual event can offer some excellent fishing in water as deep as 50 feet or more.

Seeing the Structure of the Lake

The ideal trout lake would have white marl and Chara covered shoals, strategically placed weed beds and a gradually sloping drop-off in water that is crystal clear. This would make it easy to see fish in those prime feeding areas.  Unfortunately not all lakes are clear water so a depth sounder/fish finder becomes an important tool for the lake angler. Sounders do a lot more than mark the location of fish. They tell us the depth of water we are fishing, locations of the edges of the drop-off, sunken islands and isolated shoals. They also will mark concentrations of zooplankton as well as the depth of summer thermoclines. These portable units are our underwater eyes.

Many trout fisheries are situated on irrigation reservoirs. Fluctuating water levels are a fact of life and water levels do impact where fish can live at certain times of the year. The most significant drawdown of water occurs during the warm summer and early fall months. At this time some parts of the shoal zone may become too shallow and warm for trout to occupy during daylight hours. The trout will move to the deeper and cooler water found along the edges of the drop-off. However, at night the water on the shoals will cool and oxygen levels will increase so that the trout can return to feed.

Inlet creeks and subsurface springs entering the lake are additional source s of fresh, oxygenated water that trout will congregate around or over when other parts of the lake become too warm during the summer period. Upwelling Springs are often identified by dark, irregularly shaped openings or holes that appear on the bottom of the shoal. The easiest ones to see are those occurring on Chara or marl covered bottoms.

Bathymetric Maps

Many lakes in BC have had depth soundings done to produce contour maps which show the underwater makeup of the lake. The majority of stocked lakes, which are the most popular fisheries, have contour maps and these are available to download off the Freshwater Fisheries Society of BC website Contour maps are great tools to help plan a visit to a new lake. Use them to locate the major shoals, depressions and sunken islands and other areas that typically attract fish.

Brian Chan