The majority of the North American eared grebe population migrates through the Salton Sea area.  The peak number of eared grebes visiting the Salton Sea averages 250,000.  Some winter there, while others continue south to the Gulf of California, Mexico.  Those that winter at the Salton Sea consume vast quantities of aquatic invertebrates such as primary pileworms, water boatmen, and amphipods.  On December 16, 1991, and on January 19, 1992, small numbers of dead eared grebes (approximately 20 and 180 respectively) were observed at the south end of the sea.  Approximately 150,000 were noted during another die-off along the Salton Sea shoreline between February 19-26, 1992.  The grebes were also observed exhibiting several uncharacteristic behaviors such as congregating at freshwater tributaries, repeatedly gulping freshwater, excessive preening, moving out of water onto land, and allowing close approach and/or capture.  In another die-off in 1994, 1,500 grebes were confirmed dead with estimates ranging as high as 20,000 for the total event.  The purpose of this investigation was to determine the cause of the 1992 and the 1994 die-offs.

Methods:

Dead birds were collected between March 3 and April 21, 1992, from the 40 miles of the most accessible shoreline.  Necropsies were performed in the field and in the laboratory.  Sediment samples, livers from dead, dying, apparently healthy grebes, and food items of grebes (water boatmen, pileworms, and amphipods) were collected and analyzed for selenium, arsenic, mercury, other trace metals, and organochlorines. 

Results:  

A total of 46,040 dead eared grebes were salvaged over 40 miles of shoreline.  Assuming they were spread out evenly around the Salton Sea, an estimated 150,000 eared grebes died at the Salton Sea.      In sediments, the only organochlorine found above detection limits was p,p'DDE.  Cadmium, molybdenum, tin, and beryllium were found below detection limits for all locations.  Selenium concentrations were elevated compared to national levels.   Dead and dying grebes had higher concentrations of arsenic, cadmium, chromium, zinc, and DDE than grebes assumed to be healthy.  There were no significant differences between the reference data set and the die-off data set for cadmium or mercury, but zinc and selenium concentrations were significantly higher in grebe liver tissue collected during the die-off.      Mean selenium and zinc values for pileworms and water boatmen collected during the die-off were also higher than those from samples collected in 1989. The only organochlorine found in the invertebrates was p,p'DDE found in one pileworm sample.

Discussion:

The fact that grebes salvaged during earlier stages of the die-off had similar diagnostic results to those collected during the later part suggests that the Salton Sea is the origin of the die-off.  If the source of the die-off were associated with the Salton Sea, the causative agent should have been present throughout the sea since they were flightless during the major part of the die-off and the flocks would not mix.   Contaminants found in water or food items of grebes at levels of concern included arsenic, chromium, DDE, salt, selenium, and zinc.  The mean selenium level for grebe livers found during the die-off was within the high risk adverse effect threshold.  However, this threshold was only meant to be used during the egg-laying season.  Grebes associated with the die-off had significantly higher selenium levels than birds collected at the same time of year from the Camp Pendleton reference area, the Salton Sea in 1989, and the asymptomatic grebes from the Salton Sea in 1992.  The normal symptoms of selenosis were not documented in the grebes, but selenium is known to impair immune function which would make them more susceptible to some other disease. Salt toxicosis was considered a potential cause of the illness, but eared grebes have been found to tolerate salinity levels four times greater than those found at Salton Sea. Avian botulism, salmonella, viral infection, domoic acid, and salt toxicosis were determined not to be the cause of the die-off.  Arsenic, chromium, p,p'DDE, and zinc tissue concentrations were elevated, but nothing high enough to cause acute mortality.  Tissue concentrations of mercury and selenium were high enough to cause reproductive problems, but not death.  Potential causes of the mortality are speculative and include interactive effects of contaminants, contaminant-related immunosuppresion, biotoxins, or a difficult-to-isolate manifestation of avian cholera.

For more information about the grebe die-off, read the full report: Eared Grebe Mortality in Imperial County, California, 1991-1993.  Daniel J. Audet, William Radke, Lynn H. Creekmore, Gerald Braden, Carol A. Roberts. 

For more information about the Salton Sea visit the following link.
Salton Sea National Wildlife Refuge.  Why is the Salton Sea important?

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