In South America, salmonids have been widely introduced to lakes, yet there are still many oligotrophic lakes without current abundance estimates. Catch per unit effort is a good estimator of fish abundance, but its correct implementation requires an expensive and long-term effort. Thus, there is a need to develop a rapid assessment to estimate the relative abundance of salmonids. Under well-delimited circumstances, condition indices may be a good technique to estimate fish population relative abundances. We sampled adult salmonids with gillnets from 6 oligotrophic lakes in southern Chile during the Austral summer. Our data show that as rainbow trout condition indices decrease, the abundance of salmonids increases both temporally and spatially. Because all 6 lakes are limited by food, the most likely reason for the change in the rainbow trout condition indices is change in salmonid abundance. The strong inverse relationship between condition indices of rainbow trout and the relative abundance of salmonids suggests that our model can be a useful tool for the estimation of relative fish abundance in lakes. This study provides a model that can be used to improve lake and fishery management decisions for introduced salmonids in southern South America.