The role of marine-derived contamination in the diets of California condors: Combining toxicology with stable isotope analysis

Background/Question/Methods

Lead poisoning is a major factor limiting the recovery of California condors (Gymnogyps californianus) and the principal source of lead exposure is considered lead-based ammunition.  We look beyond lead to discuss marine contamination which may pose significant barriers to condor recovery.  The threat from marine contamination will likely escalate as the growing condor population in coastal central California is predicted to increase their reliance on beach cast marine mammals as a food source and marine mammals are known to contain high levels of bioaccumulating contaminants (mercury, PCBs, DDTs). To investigate risk from marine contamination, we evaluated markers for foraging behavior (observational data, carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios) as well as mercury concentrations in condor blood. Behavioral data observed from condors in central California indicate that some proportion of their diet is composed of marine mammal carcasses, primarily carnivorous California sea lions (Zalophus californianus), whereas condors in southern California appear to forage exclusively on terrestrial animals. 

Results/Conclusions

The δ¹⁵N and δ¹³C isotope values (±SE; ‰) from whole blood collected from these birds were significantly different (t-tests, p<0.01 for both), supporting a dietary divergence between the two flocks of birds. Condors in central California (n=34) are foraging at higher trophic levels (mean δ¹⁵N = 10.1±0.2) and ingesting greater amounts of marine derived carbon (mean δ¹³C = -21.7±0.2) than condors in southern California (n=10) (mean δ¹⁵N = 7.6±0.2, mean δ¹³C = -23.7±0.4). We found a significant association between mercury exposure and stable isotope values indicative of ingestion of and contamination from marine mammals.  Mean total mercury (ng/g; ±SE) in blood samples from condors in central California (n=31) (150.8±39.5) was nearly two orders of magnitude higher than for condors from southern California (n=10) (2.0±0.2) (t-test, p = 0.04), indicating that condors foraging on marine mammals are also ingesting significantly greater amounts of mercury. We also determined the diet-tissue stable isotope discrimination factors (Δ¹⁵N, Δ¹³C) for whole blood from condors (mean Δ¹⁵N = 2.0±0.1, mean Δ¹³C = 0.3±0.2) which will enable us to utilize Bayesian isotope mixing models (SIAR) to estimate the proportions of condor diet composed of marine vs. terrestrial derived protein sources. We conclude that California condor’s struggle for survival is ongoing.  Our results highlight the challenges faced by endangered species trying to recover in a polluted environment, and they demonstrate the utility of combining behavioral, stable isotope, and toxicological analyses to determine the role of dietary choices in wildlife contamination.