The salmon… that isn’t a salmon. Brown trout (Salmo trutta L.) are one of the most widely distributed non-native fish introduced to Patagonia. They were first stocked in Tierra del Fuego by John Goodall in 1935. Shipped from Puerto Montt in Chile, 60,000 eggs survived the arduous journey to be planted on the Candelaria and McLennan rivers, both tributaries of the Rio Grande. These fish eventually found their way to the sea, attracted by the rich food supply located just offshore.
Today, sea-run brown trout complete a yearly migratory cycle just like Atlantic salmon and other salmonids that also spawn in freshwater. Sea run browns remain in the river for a period of time which ranges from 1 to 4 years until their first migration to the sea, where they will feed and grow for about 6 months before their first return to freshwater, weighing approximately 6 pounds. Researchers have found trout which have spawned as many as six or seven times. A trout that has completed 4 cycles of returning to freshwater can weigh over 20 lbs. The regularity with which these trout return to freshwater indicates that the fish face few threats. Regardless, catch-and-release fishing still rules the day.
Sea run trout are not geese, and they were not necessarily born to migrate. The role of environmental factors versus genetics on the “decision” of sea run trout to migrate is still unknown. While genetics—and likely metabolism—could play an underlying role in the development of migratory populations, studies of other fish species fail to differentiate genetically between resident and migratory individuals within a population, and in fact indicate that interbreeding often occurs between the migratory and resident individuals. In some rivers they migrate, in others they don’t. That is just the deal, but we are sure glad that the salmo trutta migrates on the Rio Grande and Rio Gallegos.