Rowell, K., Flessa, K.W. and Dettman, D.L., 2004.  Isotopic logs from the Sea of Cortez: Environmental and life-history records from Totoaba and Gulf Corvina otoliths.  Gulf of California Conference, Tucson, AZ, June, 2004.

The success of managing sustainable fisheries in the upper Gulf of California may depend on both regulating fishing practices and restoring the historic Colorado River estuary. Although fishing practices are an important cause of fishery declines and species endangerment in the northern Gulf of California, the lack of Colorado River flow to the historic 4,000 sq km estuarine portion of this federally protected area has also affected commercial as well as non-commercial marine and estuarine species. We hypothesize that river regulation has reduced brackish-estuarine habitat, which was functionally important as nursery grounds and therefore has negatively affected the economically important and endemic fishes Cynoscion othonopterus and endangered Totoaba macdonaldi. In the absence of exhaustive sampling and monitoring data, we use environmental and biological information recorded in fish otoliths (ear stones) to test the hypothesis that the species use the low salinity habitats of the Colorado River estuary during their early development. We use oxygen isotope as a tool to describe behavioral and life history shifts in both species. We found that the oxygen isotopes from otoliths indicate that both species use brackish water habitat provided by the Colorado River water during their early life history. Gaps in basic knowledge of the natural history of totoaba and Gulf corvina have limited conservation and management efforts to attempted mitigation of fishing impacts. The oxygen isotopes provide insight to the basic biology of these fish, pointing to the need to incorporate the protection of their nursery habitat.