Response of the shrimp population in the Upper Gulf of California to fluctuations in discharges of the Colorado River

Since 1935, the freshwater discharge from the Colorado River into the Gulf of California has decreased drastically, affecting the biological cycles of endemic species and other endangered species. In this study, a relationship was determined between the Colorado River freshwater discharge, the abundance of blue shrimp, Litopenaeus stylirostris (Stimpson, 1874) postlarvae, and the density of adult shrimp in the Upper Gulf of California (UGC). Data on Colorado River flow from 1904 to 2002, blue shrimp postlarvae from 1993 to 1997, and records of daily catches from pangas (small boats) of the commercial fleet operating in UGC from 1995 to 2002, were analysed. Catch per unit effort (CPUE) was used as a measure of the average daily density of adult shrimp. Two groups of CPUE and postlarvae abundance were found, with significant differences between them. The highest population density was observed in the years when the river flow was greater than 80 m³s^-. The lowest density was found for periods when the river flow was lower than 80 m³s ^-. We conclude that the response of the shrimp population is non-linear and that postlarvae abundance and commercial fleet CPUE increased during the years in which freshwater discharge was highest, possibly because the habitat volume increased, consequently resulting in increased food availability for the shrimp.