The Gulf of California is a semi-enclosed sea on the Pacific coast of Mexico, one of the most biologically diverse regions in the world with approximately 6000 reported macrofaunal species (Hendrickx et al., 2002). This is a high productivity subtropical area with approximately 258,593 km2, which is situated between the Baja Peninsula and mainland. The Gulf of California region is characterized by the presence of approximately 900 islands, 40 estuaries and lagoons. The Gulf of California ecoregion defined by the World Wildlife Fund includes the ecologically important Marismas Nacionales (National Marsh) that lies south of the Gulf proper. The Gulf and the adjacent ecosystems are populated with a multiplicity of marine mammals, birds, reptiles and a wide variety of fish and shellfish. The region supports fisheries, tourism, intensive agriculture, mining, and recently, shrimp aquaculture. These activi-
ties and the presence of around 5 millions of inhabitants constitute a serious threat to the rich and complex biodiversity of the Gulf ecoregion.