Brown trout (Salmo trutta) and sea trout (S. truttamorpha trutta) are both fish of the same species. They can be distinguished by the fact that that resident, non-migrating browns live in freshwater river systems such as the Futaleufú River, while the sea trout of Tierra del Fuego show anadromous tendencies, migrating to the ocean and returning to freshwater as massive steelhead-size specimens to spawn.
Brown trout are native to Europe and Asia but the natural distribution of the migratory forms may be, in fact, circumpolar. The fish is not considered endangered in any location. But in some cases, individual stocks are under various degrees of stress because of habitat degradation and artificial propagation leading to introgression.
Brown trout like cold (60-65 °F, or 15.5-18.3 °C), well-oxygenated waters, especially large streams in mountainous areas. Cover is important, and they are more likely to be found where there are submerged rocks, undercut banks, and overhanging vegetation.
Browns are medium-size fish, having the ability to grow to 9+ lbs. Although in most small rivers a mature browns might average 2 to 3 pounds. The sea run variations of Argentina are another story, and consistently push the 20-30+ pound envelope—a challenge for flyfishers seeking the biggest, baddest browns on the planet.
Brown trout are active both by day and by night and are opportunistic feeders. While in freshwater, diet typically includes streambed invertebrates, small fish, frogs, wayward mice, and insects flying near the water’s surface. A brown’s penchant for insect larvae, pupae, nymphs, and adult insects is what makes this trout a flyfishing favorite.