Isotopic logs from the Sea of Cortez: Environmental and life history records from Totoaba.
Rowell, Kirsten*,1, Flessa, Karl1, Dettman, David1, Román, Martha 2, Findley, Lloyd3, 1 University of Arizona, Tucson, Arizona, USA2 Instituto del Medio Ambiente y el Desarrollo Sustentable del Estado de Sonora, San Luis Rio Colorado, Sonora, Mexico3 Instituto Tecnológico y de Estudios Superiores de Monterrey, Guaymas, Sonora, Mexico
ABSTRACT- The Colorado River no longer provides fresh water to the upper Gulf of California, spawning grounds to the endemic and endangered sciaenid, Totoaba macdonaldi. In the absence of pre– and post–dam monitoring data, we use environmental and biological information recorded in T. macdonaldi otoliths (ear stones) to test the hypothesis that Totoaba use the brackish waters of the Colorado River estuary during early development. We analyzed otoliths from recent fish and compare them to pre–dam otoliths from Indian middens that are between 800– 3,500 ybp. We use oxygen and carbon stable isotopes in the otolith aragonite to describe life history and environmental changes. Stable isotopes in pre–dam otoliths indicate that the fish inhabited the freshwater plume of the Colorado River. The significant difference between the mean O18 of pre–dam and post–dam otoliths (−1.44‰ and −0.40‰ respectively) indicates that pre–dam totoaba spent their growing years in waters that were significantly less saline than today. Also, mean C13 for the pre– and post–dam fish are significantly different (0.75‰ and −1.36‰ respectively), indicating that in addition to a major shift in salinity, there has been a significant change in the carbon source. Gaps in basic knowledge of the natural history of totoaba have limited conservation and management efforts to curbing fishing impacts. The oxygen and carbon isotopes provide insight to the basic natural history of these fish, pointing to the need to incorporate the protection of their estuarine nursery habitat.
Key words: stable Isotopes, otoliths, estuary, river regulation
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