LEAD OR DDT?
California condors have been poisoned by lead, presumably acquired from eating lead ammunition fragments left in their food supply. The common belief is that the food supply in most question is hunter-shot deer. Because of this belief, one paper [1] included the suggestion that it might be safer to encourage condors to feed on marine mammals than to have them continue to eat land mammals. A basic impracticality of that suggestion is that only two of the condor populations currently being established (in Monterey County, and at Pinnacles National Park) occur close enough to the ocean to expect more than rare and incidental use of the shoreline resources by condors. But there are two other reasons to go slowly with any schemes to provide or encourage the use of marine mammals in lieu of terrestrial animals.
1. Since the condor reintroductions began, livestock and sea lions have been documented as condor food more often than deer or pigs. For example, along the central California coast between 1999 and 2010, condors were documented feeding on marine mammals 83 times (84% California sea lions), and on terrestrial mammals 23 times [2]. Livestock are shot regularly by ranchers (euthanasia), poachers, and vandals [3]. Between 1992 and 2009, the Marine Mammal Center amassed records of nearly 500 marine mammals (mostly sea lions) that had been shot. They considered this a very small sample of the actual numbers shot by vandals and fishermen trying to reduce sea lion consumption of fish [4]. Another study of about 6,200 marine mammals found stranded but still alive on California beaches from 1986 to 1998, showed that 5% had wounds from bullets or shotgun pellets [5]. As livestock shooting is not a hunting activity, and since shooting marine mammals is already illegal, neither of these sources of potential lead poisoning will be greatly affected by recent California lead ammunition restrictions. If the California bans on lead in hunting ammunition are at all successful in reducing lead levels in condors, the terrestrial food supply may become "safer" than the marine mammal supply.
2. Lead may be a threat to condors, but marine mammals have some of the highest levels of DDT, PCBs and other pesticides of any group of animals. For example, a study of pesticide residues in California sea lions along the central California coast in 2000 found that, although DDT levels have decreased markedly in the past three decades, the levels of DDT metabolites "are higher (up to 2 orders of magnitude) than current levels reported in other marine mammal species Similarly, PCB levels also appear to be higher than in several other species and geographical areas, although the difference is more moderate. In general, marine mammals from the West Coast of North America display among the highest DDT and PCB loads observed." [6].
We know that condors accumulated enough DDT metabolites by the 1960s to thin eggshells by a third over earlier samples, and to spectacularly disrupt calcium deposition in the shells. The amount of residue we found might have been enough to disrupt condor behavior and nesting success [7]. A more recent study [2] compared the eggshells of condors nesting in central California with those of southern California, and found the central California eggs significantly thinner, with the same disruption of calcium deposition we had observed earlier, and with noteworthy levels of DDE. Hatching success was significantly lower in central California, 20-40% success compared to 70-80% in southern California during the same period. The only obvious difference between the two populations is the high percentage of marine mammals in the diet of the central coast birds.
Probably there is no practical way to keep condors from feeding on marine mammals if they happen to find them, but they shouldn't be encouraged, either. With lead levels possibly as high in marine mammals as they are in deer, and with contaminant levels still so high in the marine environment, we shouldn't be too quick in exchanging one problem for another.
Notes
1. C. P. Chamberlain, J. R. Waldbauer, K. Fox-Dobbs, S. D. Newsome, P. L. Koch, D. R. Smith, M. E. Church, K. L. Sorenson, and R. Risebrough. (2005), Pleistocene to recent dietary shifts in California condors. Proceedings National Academy of Sciences 102(46):16707-16711.
2. L. J. Burnett, K. J. Sorenson, J. Brandt, E. A. Sandhaus, D. Ciani, M. Clark, C. David, J. Theule, S. Kasielke, and R. W. Risebrough. 2013. Eggshell thinning and depressed hatching success of California condors reintroduced to central California. Condor 115(3):477-491.
3. Anonymous. 1999. The emergency euthanasia of sheep and goats. Sacramento, California: California Department of Food and Agriculture. W. Jensen and J. Oltjen. 2007. Beef care practices. Davis, California: University of California Division of Agriculture and Natural Resources.
4. "Marine mammal gunshot victims," posted on the website of the Marine Mammal Center (accessed May 2016) http://www.marinemammalcenter.org/science/top-research-projects/gunshots-and-more.html
5. T. Goldstein, S. P. Johnson, A. V. Phillips, K. D. Hanni, D. A. Fauquier, and F. M.D. Gulland. 1999. Human-related injuries observed in live stranded pinnipeds along the central California coast 1986-1998. Aquatic Mammals 25(1):43-51.
6. B. J. Le Boeuf, J. P. Giesy, K. Kannan, N., Kajiwara, S. Tanabe, and C. Debier. 2002. Organochloride pesticides in California sea lions revisited. BMC Ecology 2:11.
7. L. F. Kiff, D. B. Peakall and S. R. Wilbur. 1979. Recent changes in California condor eggshells. Condor 81(2):166-172.
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