Fish without water: Validation and application of δ¹⁸O in Totoaba macdonaldi otoliths

The geochemistry of fish otoliths is a valuable tool for reconstructing environmental conditions, migrations and life histories. In this study we validate the relationship between temperature, the water oxygen isotope composition (δ¹⁸O) and otolith δ ¹⁸O for the endangered Totoaba macdonaldi, raised under controlled aquaculture conditions and in the field. This type of validation is instrumental for habitat reconstruction. By comparing δ¹⁸O values in the natal portion of totoaba otoliths from modern and pre-dam specimens ∼1000-4500 yr BP, we test the hypothesis that the totoaba used the Colorado River estuary as a nursery site before the river was over-allocated. We found that otolith δ¹⁸O could be predicted in a laboratory setting as well as in the wild. Totoaba otoliths from before river diversion had drastically lower natal δ¹⁸O values than predicted values, indicating that these differences in δ¹⁸O values are the result of a change in the water δ¹⁸O, a consequence of diverting the isotopically negative Colorado River flow from the totoaba's nursery grounds. We conclude that the Colorado River flow was a major component of the totoaba's nursery habitat before river diversions. These results are pertinent to ongoing research on this endangered fish, using otoliths to piece together important ecological and life history information.