TROUT
In rivers and streams along the Eastern Slopes of the Rocky Mountains, there are six Salmonidae species of interest to anglers (Table 1). Only two of these species spawn in the spring (cutthroat and rainbow) while the others are fall spawners. Non-native (introduced) trout species can reduce the range, and sometimes eliminate native species. For example, the introduction of brook trout is especially hard on native cutthroat populations along the eastern slopes.
| Table 1: Some members of the Salmonidae family important to fly fishing along the eastern slopes. Origin, native or introduced, can be | |||
| Common Name | Scientific name (Genus species) | Spawning Season | Location of Origin |
| Cutthroat trout | Oncorhynchus clarki | Spring | Native |
| Rainbow trout | Oncorhynchus mykiss | Spring | Introduced* |
| Brown trout | Salmo trutta | Fall | Introduced |
| Bull trout | Salvelinus confluentus | Fall | Native |
| Brook trout | Salvelinus fontinalis | Fall | Introduced |
| Mountain whitefish | Prosopium williamsoni | Fall | Native |
*Depending on the watershed, Rainbow trout can be a native species.
Salmonids, like almost all other fish species, are cold-blooded (ectotherms), that is, their internal body temperature is about the same temperature as their immediate surroundings. Therefore, a trout's metabolic rate, a measure of energy output, generally increases with increasing water temperature (Figure 2). In winter, trout require less energy input (food) because the cooler water temperature reduces their metabolic rate. Temperature can also affect growth, reproduction, and ultimately distribution. Through both behavioural and physiological response, trout reduce or "control" the negative effects of unfavourable temperature change. To avoid daily increases in water temperature, trout may temporarily move into deeper cooler water. The types and concentrations of enzymes, in the metabolic process, may change to buffer the effect of temperature change. Some fish can actually produce antifreeze molecules (glycogen) to inhibit ice formation in body fluids.
Each trout species prefers a specific temperature range depending on their life stage. For example, O. clarki, a spring spawner, holds off spawning until water temperatures reach approximately 10 C. Their eggs hatch early in the summer and fry grow rapidly in the warmer months before the onset of winter.
Fall spawners, such as S. confluentus deposit their eggs in areas where water from an underground aquifer flows to the surface (groundwater input). The eggs over-winter, protected from freezing by the outflow of groundwater, and eggs hatch early in spring. Fry have several months of warmer temperatures to grow before temperatures drop in the fall.
Most trout species presented in Table 1 prefer maximum summer temperatures between 10 C and 20 C. These trout can withstand higher temperatures (20 to 24 C) if exposure is temporary (usually < 1 or 2 hours) and infrequent (usually no more than 2 or 3 consecutive days). Rainbow trout, however, are found in temperatures as high as 24 C that is sustained over longer periods.
Figure 1: General relationship between metabolic rate and water temperature for ectotherms (e.g. trout and insects). As water temperature increases, metabolic rate increases to an optimum. When temperature is higher than the optimum the metabolic rate drops until the maximum tolerable temperature is reached. Prolonged exposure to temperatures at, or higher than the tolerable maximum, will kill an organism.
There are several external features of a salmonid body used for identification. Of special note are the branchiostegal rays (7 to 20), adipose fin, vertebrae (57 to 75), and cycloid scales. The lateral line has special sensory structures able to detect change in water pressure and temperature (Figure 2).
Scales cover the entire body surface and function as a protective barrier. If scales are removed, bacteria and mold can attach and contaminants are absorbed through the skin, thereby, stressing the fish. Depending on the number of scales removed, trout can become weak and die. Therefore, anglers must be very gentle when handling fish. Look at your hands after you release the fish, if you see scales you are too rough. There are several release techniques used to reduce fish stress. First, crimp the barb on the hook. Second, use a net to hold the fish. Third, do not keep the fish out of the water for more than a couple seconds. Finally, release the fish in calm water after it has recovered.
Figure 2: Basic features of a typical salmonid (lateral surface).
A trout's range of vision includes regions of both monocular and binocular vision (Figures 3 and 4). Due to the arrangement of rods (dim light sensitive) and cones (colour and detail sensitive) in the trout's eye, it can effectively see in most directions at once (both binocular and monocular regions). Given the placement and limited range of movement of their eyes, there is also a region in which a fish cannot see - a blind spot.
Salmonids are unable to hold position while oriented downstream (with the flow) because of their body design. Their large caudal fin provides propulsion while relatively small pectoral and pelvic fins provide stability and direction. Therefore, when stationary, trout always face into the flow. For these reasons (blind spot and instream positioning), fly-fishing in a downstream to upstream direction is a very good strategy.
Figure 3: Trout's range of vision showing regions of monocular and binocular vision, as well as the region in which it can not see (blind spot). Dorsal view.
Figure 4: Trout's range of vision. Lateral view.
A trout's diet is usually quite diverse and varies depending on food availability, stage of life (size of the trout), and degree of competition. Although some trout species are more likely to consume specific food items, they only have a choice of what is available at the time. Some food types include:
Insects
Aquatic - such as stoneflies, caddisflies, mayflies, craneflies, and midges.
Terrestrial - such as ants, beetles, and grasshoppers.
Fish
Their own and other species (e.g. sculpins and dace), and can include all stages of life - eggs, fry, juveniles and adults.
*During a research study for the University, I found a duckling in the stomach of a 12" rainbow! |
Other Animals
Leeches, shrimp, snails, mice and birds (ducklings)*.
It is important to keep in mind that except for the period during spawning and during the coldest months in winter, trout are not only interested in feeding, they are actively doing so. A perceived lack of interest to a food source provided by the angler is only an indication of preference by the trout.
The stages of a trout's life cycle are relatively simple, going from egg to juvenile to adult. However, the time required to complete the life cycle (egg to adult to egg) is highly variable. There can be tremendous differences between species with respect to the time required to complete growth and development. For example, bull trout may take six or seven years to reach sexual maturity, whereas rainbow trout can require only two or three years. There is also large growth and developmental differences within a species, with factors such as water temperature, food quality, and food availability playing a major role. Trout in a nutrient rich system grow and develop much faster than trout in a nutrient poor system. This relationship is clearly illustrated with Bow River trout populations, where trout that are located upstream of Calgary (a relatively nutrient poor section) are generally smaller than trout downstream of Calgary (a relatively nutrient rich section).
Habitat preference varies between species, and changes with life stage and time of year. Brown trout adults prefer deeper pools, large debris and undercut banks whereas rainbow trout adults prefer shallower areas with boulders spread throughout. To avoid predation by larger trout, rainbow fry (juveniles < 1 year old) move close to shore in very shallow water and often near small bundles of debris. Trout congregate in deeper pools over the winter to avoid being trapped in an area that may temporarily freeze to the bottom.
Trout can be territorial with the largest (usually the most dominant) taking prime locations for resting, feeding and mating. Typically, the largest trout will defend its position near the "head" of a feeding lane. This ensures them the first opportunity for a food item as it passes by.