Colouration: green to brown basic colouration with a distinctive marbled pattern (called vermiculations) of lighter shades across the flanks and back and extending at least to the dorsal fin, and often to the tail. There is a distinctive sprinkling of red dots, surrounded by blue haloes, along the flank. The belly and lower fins are reddish in color, the latter with white leading edges. Often the belly, particularly of the males, becomes very red or orange when the fish are spawning. The species reaches a maximum recorded length of 86 cm (33 in) and a maximum recorded weight of 9.4 kg (14 lb). It can reach at least seven years of age, with reports of 15-year-old specimens observed in California habitats to which the species has been introduced.One such intergeneric hybrid, between the brook trout and the brown trout (genus Salmo) is the tiger trout. Tiger trout occur very rarely naturally but are sometimes artificially propagated. Such crosses are almost always reproductively sterile. They are popular with many fish stocking programs because they can grow quickly, and may help keep rough fish populations in check due to their highly piscivorous (fish-eating) nature.

A less frequent natural hybrid is the splake, a hybrid between the brook trout and lake trout. Although uncommon in nature, some jurisdictions artificially propagate splake in substantial numbers for planting into brook trout or lake trout habitats. An example would be in Ontario, where both F1 splake and a fish known as the lake trout backcross have been planted for several years.[6] The backcross is the result of an F1 splake male being crossed with a female lake trout (i.e., 75% lake trout and 25% brook trout). Although splake were first described in 1880, Ontario began experimenting with the hybrids in the 1960s in an effort to replace collapsed lake trout stocks in the Great Lakes. Due to mediocre results, the experiment never really progressed beyond Georgian Bay. The theory was that splake would grow more quickly and mature sooner than lake trout with the hope that they would be able to reproduce before being attacked by the invasive sea lamprey. Unfortunately, although splake are relatively unusual among hybrids in that they are fertile, fertility in nature is behaviourally problematic — very few natural progeny are produced by introduced splake populations. After some experimentation in the late 1970s, stocking in the Great Lakes and, especially, in Georgian Bay, was converted entirely to the so-called lake trout backcross in the early 1980s. Although the backcross program did succeed in creating some localised angling opportunities, it never achieved any degree of success in terms of natural reproduction — the backcross was only marginally better at reproducing than was the F1 splake. The F1 splake has proved to be a success, however, in providing angling opportunities in smaller lakes and most of the planting of splake in Ontario now goes to those situations. There are two general cases. In the first case, former brook trout waters which have become infested with spiny-rayed fish to the point where they no longer produce brook trout are stocked with splake. The splake grow more quickly than do wild-strain brook trout and become piscivorous at a younger age and, hence, are more tolerant of competitors than are brook trout. In the second case, relatively small lake trout lakes that experienced poor recruitment due to insufficient deep-water juvenile lake trout habitat will support fairly good splake fisheries since splake are less dependent on extreme deep water than are the lake trout and they grow more quickly, providing a better return to anglers. In both cases, due to the behavioural sterility of splake, all such fisheries are entirely dependent on artificial propagation.

Brook-trout populations depend on cold, clear, well-oxygenated water of high purity. As early as the late 19th century, native brook trout in North America became extirpated from many watercourses as land development, forest clear-cutting, and industrialization took hold. Streams and creeks that were polluted, dammed, or silted up often became too warm to hold native brook trout, and were colonized by transplanted smallmouth bass and perch or other introduced salmonids such as brown and rainbow trout. The brown trout, a species not native to North America, has replaced the brook trout in much of the brook trout's native water. Brook trout populations, if already stressed by overharvest or by temperature, are very susceptible to damage by the introduction of exogenous species. Many lacustrine populations of brook trout have been extirpated by the introduction of other species, particularly percids but sometimes other spiny-rayed fishes. In addition to chemical pollution and algae growth caused by runoff containing chemicals and fertilizers, air pollution has also been a significant factor in the disappearance of brook trout from their native habitats. In the United States, acid rain caused by air pollution has resulted in pH levels too low to sustain brook trout in all but the highest headwaters of some Appalachian streams and creeks.[7]. Brook trout populations across large parts of eastern Canada have been similarly challenged; a subspecies known as the aurora trout was extirpated from the wild by the effects of acid rain. Today, in many parts of the range, efforts are underway to restore brook trout to those waters that once held native populations, stocking other trout species only in habitats that can no longer be recovered sufficiently to sustain brook trout populations.