Otolith Diaries: Isotopic Trails Tell Ecological Tales of a River Diversion or Overfishing?

In the 1950-1960’s the construction of dams and diversions significantly reduced the Colorado River’s flow into the Gulf of California.  Nearly simultaneously, the fishing industry in the Gulf of California began to boom.  Although quantifying the ecosystem responses to these different pressures is important to ecosystem management, it is very difficult without baseline natural history information before anthropogenic disturbances.  In effort to assess the bottom-up effect of no fresh water delivery to the nursery habitat and the top-down effect of over-fishing on four economically important species of Sciaenids (Totoaba macdonaldi, Cynoscion othonopterus, C. parvipinnus, and Micropogonias megalops), we turn to growth and geochemical records in ancient otoliths (1000- 5000ybp).  We use a suite of stable isotopes (δ¹⁸O, δ¹⁵N, and δ¹³C) to document pre-disturbance use of brackish water habitat provided by the river, river derived nutrients, and trophic position. Our isotopic results show clear behavioral differences between species and a past/present comparison suggest that the Colorado River was an important nursery habitat for three of the four species.  Nitrogen isotope ratios in otoliths of Totoaba, a top predator, suggests that one full trophic level has been removed from the food chain since human alteration of the Colorado River estuary.  Life history analyses suggest significant changes in growth rates of two species since pristine conditions. These findings provide novel insights about the influences of two major anthropogenic pressures in the upper Gulf of California.