Though most of the Rio Grande is on the Chilean side of the Tierra del Fuego border, nearly 80% of the spawning of sea run trout occurs on the Argentina side of the island. In fact, most of that is in the first 35 miles above where the Rio Grande enters the Atlantic Ocean. Above that point, the pools of the river become increasingly shallow, with progressively more marginal spawning habitat, and fewer and fewer sea trout. Small resident brown trout thrive in the main river tributaries, and there is a very healthy population of rainbow trout in the Chilean headwaters.
Less than a decade ago, a team of the most renowned fishery biologists in the Americas was charged with the daunting task of determining exactly how many sea run brown trout are actually in the Rio Grande.