Heavy metal concentrations in mojave desert tortoises (Gopherus agassizii) related to a mitigation translocation project, Ivanpah Valley, California, USA

The Mojave Desert Tortoise (Gopherus agassizii) is listed as threatened under the U.S. Endangered Species Act because of subsidized predation by other species, loss and degradation of its habitat owing to human activities, and disease. Potential exposure of toxic substances on G. agassizii that possibly impede recovery, however, have not been thoroughly investigated. To quantify concentrations of several heavy metals and examine possible adverse effects of heavy metal toxicity on G. agassizii, we analyzed blood samples using the Dried Blood Spot (DBS) method and soil samples from their locations in the Ivanpah Valley, California, USA. In most cases, heavy metal concentrations in blood never or rarely exceeded minimum detection levels (typically, 0%-7% of samples in a given season). In soils, several heavy metals (e.g., arsenic, lead, and thorium) exceed average crust composition, but none exceeded soil health guidelines. Furthermore, lead, selenium, iron, and arsenic concentrations were lower than, or within, published ranges for turtles, reptiles, and other vertebrates. We found a positive relationship between survival and selenium and iron concentrations but no relationship between metal concentrations and health indicators such as body condition and disease prevalence. Our results suggest that G. agassizii in our study area were not exposed to toxic levels or suffered adverse effects of heavy metals. The DBS method is minimally invasive and effective for the collection of blood samples from G. agassizii. Further analyses should explore how well samples collected by the DBS method reflect metal concentrations in other tissues.