The
Salton Sea has been the site of several fish and bird die-offs in recent years.
In 1996 a major outbreak of Type C avian botulism occurred. There were
over 14,000 bird deaths including over 8,500 American white pelicans and over
1,100 brown pelicans. This outbreak was different from others that have
occurred in the United States because it happened to fish-eating birds.
The purpose of this study was to determine if exposure to contaminants may have contributed to the die-off in brown and white pelicans.
Methods: Specimen birds were collected randomly from the dead birds brought into the Salton Sea National Wildlife Refuge. Breast muscle, liver tissue samples, and brain tissue were collected from the birds. The muscle tissue was analyzed for organochlorine compounds, the liver tissue was analyzed for inorganics, and the brain tissue was analyzed for cholinesterase.
Results and Discussion: The organics detected included dieldrin, polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), and its metabolites. Dieldrin was detected in 9 of 10 brown and white pelican samples. The concentrations found have been shown to have no measurable effects on reproduction, so it does not appear to be significant relative to the die-off. The PCBs detected were also below levels associated with chronic poisoning and reproductive problems so they are not of major concern in the Salton Sea ecosystem. Of the DDT metabolites detected, o,p'DDT and p,p'DDT were not in high enough concentrations to cause adverse impacts. Low levels of o,p'DDE was found in the majority of the muscle samples. These concentrations would only contribute slightly to the impacts associated with this group of compounds and are not likely to have been an independent factor in the die-off.
The
levels of p,p'DDE residue were similar to concentrations measured while
DDT was still in use. It is possible that these birds are being exposed
to DDT in Mexico. The concentrations of DDE detected though were below
levels associated with chronic poisoning and reproductive problems.
However, additional studies would be required to determine if DDE is accumulating
in the eggs and impacting reproduction. The following inorganics were detected, but at low enough levels that they are not thought to cause harm: aluminum, arsenic, barium, boron, cadmium, chromium, copper, magnesium, manganese, molybdenum, nickel, strontium, vanadium, and zinc. Iron was detected in all liver samples in concentrations which exceeded those found in other waterfowl. Since iron is considered to be an innocuous trace element it was unlikely to have contributed to the die-off. However, the significance of the extremely high values in some of the white pelicans does warrant future investigation. There also was a correlation with mercury, so it is possible that the sources of these two may be related. Mercury was detected in high concentrations in 15 of the brown pelicans and all ten of the white pelicans. The effect may not be seen, though, because interactive effects can occur between selenium and mercury. By association with high levels of selenium, mercury can accumulate in liver tissues in mammals without exerting any apparent toxicity. On the other hand, mercury toxicity can be increased by pesticides such as DDE and elevated temperatures. Temperatures during the die-off exceeded 100 F on a daily basis, however, the birds did not display symptoms of mercury poisoning so mercury does not appear to have been a major causative factor in the die-off event. Selenium was detected in all brown and white pelican samples. Eleven of the brown pelicans and seven of the white pelicans exceeded the reproductive impairment threshold, however species that prefer more saline environments do appear to have a higher selenium tolerance. Selenium concentrations and effects can also be modified by the presence of other trace elements.
Greater
impacts are seen as a result of a combination of boron and selenium in the
diet, particularly if protein consumption is low. Arsenic alleviates
some of the effects of selenium. Both boron and arsenic are available
in this system, but the relative magnitude of these opposing effects is
not understood for these species. The information does not clearly
indicate whether the selenium concentrations found are problematic for pelicans
or not. The only conclusion that can be made is that selenium contamination
may have been a contributing factor in the overall health and susceptibility
to the botulism toxin of the birds. Acetylcholinesterase:
Brain cholinesterase activity
measurements were taken on brain samples from the ten brown pelicans.
The acetylcholinesterase assay includes a reactivation step which can remove
bound organophosphates and restore cholinesterase activity. Five of the
samples tested showed reactivation indicative of organophosphate exposure.
Exposure to cholinesterase inhibiting pesticides such as organophosphates
can result in incoordination, muscular weakness and slight paralysis in
birds which are similar to the symptoms of botulism. High temperatures
also tend to exacerbate the impacts of pesticide exposures and compound
the difficulties in thermoregulation experienced by birds exposed to the
botulism toxin. Botulism toxin is lethal in nanogram quantities so
this probably minimized the effects of organophosphate pesticide exposure
relative to the effects of the toxin.
Find out more by reading
the following full report:
Carol A. Roberts. Contaminants in Pelicans
Collected During the Avian Botulism Event at the Salton Sea in 1996.
US Fish and Wildlife Service. October 1997
Return to the Carlsbad Field Office Reports
For more information visit the following links:
Salton Sea National Wildlife Refuge. Why is the Salton Sea important?
Salton Sea National Wildlife Refuge. Summary of 1996 Botulism Event.
Visit the: Pacific Region - Environmental Contaminants-Investigating and Monitoring
Visit the: USFWS- Pacific Region Ecological Services Home Page
