Large marine regions characterized by unique bathymetry, hydrography and productivity, within which marine populations have adapted reproductive, growth, and feeding strategies constitute large marine ecosystems (LME) (Sherman and Alexander, 1986). The Gulf of California a semi-enclosed sea off the western coast of Mexico qualifies as a LME as it presents physical and biological characteristics not found in the adjoining Pacific. This area sustains the largest commercial fishery in the country which is the sardine fishery. the sardine population resident in the Gulf of California has coupled its reproductive and migration strategies to the environment of this region. Fluctuations of the sardine stock in the Gulf of California are caused by environmental changes and human activities within the Gulf. This paper presents the major physical and biological components of the Gulf of California and identifies the major source of change for the sardine fishery found in this LME