Contaminant residues and declines of the Cascades frog (Rana cascadae) in the California Cascades, USA.

Populations of Cascades frogs (Rana cascadae) have declined precipitously in the Mount Lassen area, but remain abundant in the other half of their California range in the Klamath Mountains. To evaluate the role of contaminants in Cascade frog declines, we sampled sediment and frog tadpole tissue at 31 sites where Cascades frogs had disappeared and sites where Cascades frogs are still present across the Lassen and Klamath regions. Pacific chorus frogs (Pseudacris regilla) were tested and used as surrogates for residue concentrations in Cascades frogs. We analyzed a total of 79 tadpole samples for 73 semivolatile contaminants including pesticides, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). The most frequently detected residue was endosulfan sulfate, followed by dacthal, chlorpyrifos, PCB 187, endosulfan II, trans-chlordane, and trans-nonachlor. Chorus frogs had similar residue concentrations as Cascades frogs for most but not all chemicals, indicating that chorus frogs can serve as a reasonable proxy for chemical concentrations in Cascades frogs. None of the contaminants in tissue or sediment had significantly higher concentrations at sites where Cascades frogs have disappeared than at sites where Cascades frogs are still present. We found no evidence to support the hypothesis that the contaminants analyzed have contributed to the decline of Cascades frogs in northern California, although we were able to analyze only a handful of the over 300 pesticides currently used in the area.