Partial Migration in a Robust Brown Trout Population of a Patagonian River

Life history plasticity, particularly partial migration (i.e., coexistence of resident and anadromous forms), probably plays a major role in the ability of brown trout Salmo trutta to dominate food webs after introduction into Patagonian rivers. We documented brown trout establishment, distribution, and life history variation in relation to habitat features in the Rio Grande, Tierra del Fuego, Argentina. Results support the hypothesis that Rio Grande brown trout developed an anadromous life history form (“sea trout”) two decades after introduction in relation to localized food limitation as the population expanded throughout the basin. Tributary streams with only resident fish were narrower and had significantly higher levels of specific conductance and soluble reactive phosphorus than streams that were inhabited by both resident and anadromous brown trout. Invertebrate biomass was two to three times higher and juvenile brown trout density was significantly higher in resident‐only sites than in streams with both life history forms. Density‐dependent growth was not evident and juvenile condition was higher in resident‐only streams. These findings suggest that food availability influences the individual decision to migrate, although genetic and other factors are probably also involved. Mark–recapture analysis showed that approximately 38,000–55,000 adult sea trout returned annually to the Rio Grande in 2006 and 2007, making it one of the most robust sea trout populations in the world. However, native fishes have apparently declined to near extirpation as the brown trout population has expanded.