These trout are dark gray with numerous light gray spots on the sides, both above and well below the lateral line. The tail is deeply forked.
Lake trout are the largest of the charrs, the record weighing almost 46.3 kg (102 lb). They were fished commercially in the Great Lakes until lampreys, overharvest and pollution extirpated or severely reduced the stocks. Commercial fisheries still exist in some smaller lakes in northern Canada.
Lake trout spawn in September in lakes, in waters 1 to 120 feet deep. They may spawn in pairs or several may spawn as a group. The eggs are quite large (1/2 inch in diameter), and are spread over boulders, or gravel. The eggs are fertilized as they fall between the rocks. Incubation may require several months; hatching has at least begun by March. The juvenile fish grow very slowly. Most of the lake trout caught are less than 10 pounds but some become large, exceeding 40 pounds. Lake trout first mature as a 6- or 7-year-old fish and may live up to 35 years.
Lake trout are dependent on cold, oxygen-rich waters. They are pelagic during the period of summer stratification in dimictic lakes, often living at depths of 20–60 m (60–200 ft).
The lake trout is a slowly growing fish, typical of oligotrophic waters. It is also very late to mature. Populations are extremely susceptible to overexploitation. Many native lake trout populations have been severely damaged through the combined effects of hatchery stocking (planting) and overharvest.
Lake trout have been known, very rarely, to hybridise in nature with the brook trout, but such hybrids are almost invariably reproductively sterile. Hybrids, known as "splake" are also artificially propagated in hatcheries and then planted into lakes in an effort to provide sport fishing opportunities.
Young lake trout feed on freshwater shrimp and other aquatic invertebrates. Larger lake trout are usually predators and eat other species of fish. In Idaho, kokanee and sculpins are often eaten by lake trout.
In early spring and fall, lake trout frequent shallow areas. When the water is fairly uniform in temperature, they can be found at all depths. As upper water temperatures increase in summer, mackinaw seek cooler, well-oxygenated levels deeper in the lakes.
Lake trout prefer water temperatures of 50 degrees F. or less. As a result, the best fishing for this species is in the early spring right after ice out or in the late fall when the surface water is cold. The lake trout are a prized game fish, and may be taken be fly casting, spin or bait casting. Streamer flies, spoons, spinners and plugs are effective lures if fished close to the bottom.
The most common method is trolling with large spoons. Lake trout are extremely sensitive to water temperature and some local knowledge of the area being fished is a great advantage.