Lethal Parasite Prime Suspect In Fish Kills At Salton Sea

September 10, 1997

Investigators seeking causes of recurrent massive die-offs of fish and water birds at California's Salton Sea may have identified a principal culprit, according to Dr. Milton Friend, director of the National Wildlife Health Center.

The finding comes as a new die-off of tilapia, the predominant fish species in the Salton Sea, has been reported with more than one million dead fish over a three-mile stretch at the north end of the Sea, Department of the Interior officials said.

Drs. Tonie Rocke and Lynn Creekmore, scientists from the US Geological Survey's National Wildlife Health Center (Madison, Wis.), conducted a field investigation in late August that focused on collecting sick or freshly dead fish from locations where most of the bird mortality was occurring. In collaboration with NWHC studies, Dr. Jan Landsberg, a research scientist in aquatic health at the Florida Department of Environmental Protection, examined gills from 23 fish collected at the Salton Sea for parasites. She found 22 to be infested with moderate to high levels of a lethal parasite of warm-water marine fish.

The parasite, a dinoflagellate known as Amyloodinium ocellatum, is recognized as a persistent disease-causing agent that causes serious mortalities in fish-farming facilities and public or home marine aquaria. Under closed conditions, such as aquaria and apparently also in the Salton Sea, the investigators reported, parasite levels can build up to extremely high levels in fish. Healthy fish in aquaria can die after only 12 hours of exposure to Amyloodinium.

Like other USGS science centers, the National Wildlife Health Center has a primary responsibility for assisting the public land managers of the Interior Department. "Because a national wildlife refuge occupies a significant portion of the Salton Sea, the Health Center's assistance was sought and delivered," said USGS Chief Biologist Dennis B. Fenn.

"Over the last five years", Dr. Friend said, " the Salton Sea has been the site of several major die-offs. Disease problems in birds have included avian cholera, Newcastle disease, avian botulism and undiagnosed mortality in eared grebes. Disease problems in fish have been less well defined, but infections with a type of bacteria known as Vibrio spp. have been identified as the potential cause of some of the massive fish mortalities".

In late summer and early autumn of 1996, a large mortality event in birds was found to be caused by avian botulism. This outbreak resulted in the death of more than 14,000 fish-eating birds including 1,400 endangered California brown pelicans and 10-12% of the west coast population of American white pelicans. At the same time, a die-off also occurred in tilapia. These fish were suspected to be the source of the botulism toxin for the birds.

In August 1997, another outbreak of botulism occurred in fish-eating birds. Over 605 carcasses including 234 gulls, 138 brown and white pelicans, and 135 herons and egrets were picked up. Again, sick and dead tilapia were observed at the same time.

The suspect parasite is found worldwide and infects more than 100 species of marine fish in North America alone. In wild fish, the number of parasites per fish is typically very low, and they do not usually die from the infestation.

When present at levels as high as those detected in tilapia from the Salton Sea, the parasite impairs respiratory function and can suffocate the fish. Diagnosis is based on finding the attached feeding stage of the parasite present in gill scrapings. Since the life cycle of Amyloodinium can be completed in less than one week at high temperatures and in high saline conditions, like those currently present at the Salton Sea, it is expected that massive and lethal infestations can develop rapidly.

Now that the parasite is present in the Salton Sea, and is able to reproduce without control, persistent, chronic die-offs of fish may be expected to continue, the scientists said. High salinity is optimal for this parasite; it does not live in freshwater or in low salinity conditions. The connection between this parasite, other bacterial infections in fish and the occurrence of botulism in fish-eating birds is still under investigation. Along with environmental stressors and other disease-causing agents identified in fish and birds, Amyloodinium represents another threat to the overall health of the Salton Sea ecosystem.

The USGS-managed Wildlife Health Center is the foremost wildlife diagnostic and investigative facility of its type, devoted to identifying causes and possible management responses for episodes of death or debilitation among free-ranging wild creatures throughout the United States and -- on a consulting basis -- other nations.

US Geological Survey

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