Local fishers' knowledge (LFK) obtained from 49 fishers in the upper Gulf of California indicates that fishery resources have declined by at least 60% over the past 50 years, most likely due to overfishing and environmental changes associated with upstream damming of the Colorado River. LFK can provide supporting evidence of trends and changes. In marine ecosystems, for example, relative abundances obtained from LFK for several commercial and non-target species are closely correlated with stock assessment biomass, fishing effort, and landings reported by the Mexican Government. Rapid shifts in perception of the degree of degradation of this ecosystem, as well as greatly reduced nutrients and river flows (over just a few decades), should act as a red flag to the Mexican Government and management agencies. It is crucial for the restoration of this ecosystem that young fishers and the Mexican public are able to visualize previous states of their local ecosystems.