Sea-Run Brown Trout (Salmo trutta) are one of the most widely distributed non-native fish introduced to Patagonia and Tierra del Fuego. 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 and went on 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.
Today, Sea-Run Brown Trout complete a yearly migratory cycle just like Atlantic Salmon and other salmonids that spawn in freshwater. Sea-run browns remain in the river for a period of time that ranges from 1-to-4 years until their first ocean migration, where they will feed and grow for about 6 months before their first return to freshwater, typically weighing approximately 6 pounds. Researchers have found trout that have spawned as many as six or seven times. A Sea-Run Brown that has completed four cycles of returning to freshwater can weigh more than 20 pounds. The regularity with which these trout return to freshwater indicates that the fish face few threats. Regardless, catch-and-release fishing is strictly followed to ensure these fish always return.
The Río Grande’s Sea-Run Brown Trout are different. Mostly because they grow to unbelievable sizes. The average size varies but is usually over 7 pounds. Fifteen-pound fish are common and every week of the season 20-pound fish are released—yes, you read that right, every week. You have at least a one-in-five chance of catching a 20-pound fish every time you put a fly in the water. Our catch records go back to 1984 and these Browns get huge. That’s a simple truth.
Sea-run Brown Trout are not born migrators. The role of environmental factors versus genetics on the “decision” 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’s just the deal, but we are sure glad that the Salmo trutta migrates in the Río Grande and Río Gallegos.
