Brown trout were introduced into the Rio Grande in the 1930s where they proliferated throughout the catchment.  By the 1990s, the population became world renown for sport fishing as large body size associated with anadromy eventuated.  We observed that fish were exclusively resident in several tributaries whereas sea-runs and residents co-occurred in the rest of the catchment.  We hypothesized that partial migration characterized this population as a result of habitat limitation such that fish rearing in locations with insufficient food resources were more likely to exhibit migratory behavior than fish rearing in locations with more food resources.  Sites supporting only resident fish were narrower, exhibited significantly higher specific conductance, and contained vastly higher forage.  In resident (as opposed to anadromous) habitat, significantly higher juvenile growth occurred as determined by scale analysis.  We concluded that anadromy was predominate in the population but partial migration, as mediated by food and space limitation, allowed brown trout to fully colonize and dominate the Rio Grande.  Native galaxiids, an obvious prey item during invasion, were functionally extirpated.