A good trout stream has diversity. Habitat requirements are different for adult trout, juveniles, fry and eggs. While different life stages require different habitats, all trout need clean, cold water. Above all, this means having enough water in the stream to hold trout through all life stages, from spawning to adulthood. It also means keeping the water at temperatures that can support trout. Different species of trout have different temperature tolerances. For most adult trout, water temperatures in the 70s (°F) are lethal. While trout may survive higher temperatures for a short time, they cannot survive sustained exposure to these temperatures. Generally, temperatures in the 50s and low 60s (°F) are best for trout growth. Developing eggs in the gravels are less tolerant of high temperatures than adult trout.
Dissolved oxygen is a key element. Trout are active fish, requiring a plentiful supply of oxygen. Oxygen is introduced to a stream from the atmosphere by the splashing of water (for example, over riffles) and from photosynthesis in aquatic plants and algae. The amount of oxygen water can hold varies with water temperature and elevation. For most coldwater streams, oxygen levels of six parts per million (6 ppm) are needed for good trout growth and survival. Good trout streams also offer a plentiful food supply.
Aquatic invertebrates are the principle food for most trout, though they will feed on smaller fish and terrestrial insects as well. Trout food organisms depend, in turn, upon aquatic plants (including algae as well as rooted plants) and plant matter that has fallen into the stream from riparian vegetation.
Clean bottom gravel (free of excess silt and other fine sediments) is also important for many aquatic invertebrates. Clean gravels are important for trout spawning habitat, as well. If silt clogs spawning gravels, trout eggs may be suffocated or fry trapped in the gravels. Well-aerated water should pass through spawning beds to provide oxygen and remove wastes. Spawning areas should also be free from large-scale scouring and bed movement, or the delicate eggs and sac-fry may be crushed. To meet this variety of needs, spawning beds are often found in gentle glides, at the tail ends of pools (upstream from a riffle), and/or over in-stream springs where fresh water wells up through the gravels. There are both Life Cycle and Seasonal Habitat Requirements.
Just as habitat needs vary at different points in a trout’s life, they also change over the course of the year. In areas with cold winters, trout seek out areas (such as deeper pools) that will not freeze to the bottom. Spring-fed streams, which maintain a constant water temperature, also make good over-wintering habitat. In areas with heavy spring runoff, trout need habitat that provides shelter from the extreme flows-for example, in secondary channels, backwater pools, shallows along the stream’s edge, or protected areas behind in-stream debris. In areas with low summer flows, trout congregate in larger pools that maintain sufficient depth despite the low water levels.
This review highlights only some of the connections between trout and their environment.