Among the 34 species of marine mammals that inhabit the Gulf of California, the only pinniped that reproduces in this region is the California sea lion Zalophus californianus. Of the 13 breeding stocks, the most studied is situated on Los Islotes, a small island located in the Bahía de La Paz, the largest bay in the Gulf. Changes in the demographic density of this stock from 1979 to 2004 were analyzed per age and sex group with the SIRONAS routine (Simulation ROutine of Natural Stocks), using demographic rates obtained from the CODYS model (COhort DYnamics Simulation). Differences between the model data base and the data base of annual censuses provided an accurate demographic variability per age and sex group, since SIRONAS is a routine that generates a virtual population over a period of time. These differences in demographic rates suggest the action of environmental forces with variable intensity and provide for the possibility of local movement patterns among breeding colonies. The SIRONAS routine is the complement of CODYS for a quasi-real demographic analysis for the sea lion in the Gulf of California.

annual censuses, breeding colony, demographic density, Gulf of California, SIRONAS, Zalophus californianus.