First Known Sightings of "Withering" Abalone Bacteria in Northern California
April 1, 2002
Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334
The abalone specimen on the left has withering disease. Note the severe atrophy -- or withering -- of its foot muscle. The specimen on the right is healthy.(Photo credit: California Department of Fish and Game)
A researcher funded by California Sea Grant has discovered the first known sightings of the bacterium that causes "withering" abalone disease in waters north of San Francisco. Previously, the bacterium, and the withering disease it causes, had been observed only in coastal areas south of San Francisco.
Dr. Caroline Friedman, a senior fish pathologist with the California Department of Fish and Game and an associate adjunct professor at University of California at Davis, discovered two areas north of San Francisco where tissue samples from red and black abalone were shown to be infected with the bacterium. These were in Crescent City near the Oregon border and Van Damme State Park in Mendocino County.
Although none of the abalone showed symptoms of withering syndrome, it is feared that the infected abalone may disperse the bacterium via their feces, increasing the potential for an outbreak later.
Once a valuable fishery, abalone in Southern and Central California have been decimated by commercial and sport fishing, and more recently by withering abalone disease. Some populations of black abalone have declined nearly 99 percent since the disease was first observed in 1986 in the Channel Islands off Santa Barbara.
Withering syndrome is caused by the bacterium Candidatus Xenohaliotis californiensis. This bacterium attacks the lining of the digestive track, obstructing the production of digestive enzymes. In response, abalone consume their own body mass, causing "withering" of the mollusk's muscular foot. This atrophy of the foot muscle impairs their ability to adhere to rocks, making them vulnerable to predation. Abalone not eaten by predators usually end up dying by starvation.
For reasons not yet well understood, some abalone can be infected with the bacterium without developing the disease. It is believed, however, that changes in environmental conditions, such as warmer than normal water temperatures, may induce the disease in abalone that already harbor the bacterium. For this reason, the spread of the bacterium could lead to further declines in abalone populations, especially during El Nino events, when water temperatures rise.
An abstract of Dr. Friedman's work was recently published in the Journal of Shellfish Research. Dr. Friedman said that she and colleagues plan to conduct a second survey to resample abalone beds and analyze their tissue in the summer of 2001. The first set of abalone samples were collected from 13 sites in Northern and Central California between December 1998 and October 2000.
Explaining the significance of the survey's results, Dr. Friedman said that "the only healthy abalone stocks in California are found in Northern California. Aside from its ecological importance, the north coast red abalone sport fishery is also an important tourist attraction. Conservation of these stocks are critical."
The source of the bacterium in northern California is not known. But Dr. Friedman said that human activity may be to blame. Both Crescent City and Van Damme State Park received abalone "out plantings" in which cultured baby abalones, or abalone seed, were released for stock enhancement.
These out plantings were conducted to revive ailing abalone beds, before it was known that withering syndrome is caused by Candidatus Xenohaliotis californiensis. Crescent City is also near an aquaculture farm, which has historically imported abalone from farms now known to be in areas where high numbers of abalone have had withering syndrome.
Abalone are large mollusks, closely related to snails. Unlike mussels or clams, which are bivalves, abalone have a single shell. These can reach 12 inches in diameter. Beneath their shell is a large muscular (and edible) foot. Abalone locomote on this foot, much as do snails in your garden.
There are about 70 species of abalone in the world. Seven are native to California. These include the red, green, pink, pinto, flat, white and black abalone. Some of these species, such as the black abalone, dwell in the rocky intertidal zone, where animals are exposed to air at low tides. Other species live permanently beneath the sea in deeper waters.
All of California's seven abalone species have been severely depleted. Some biologists now doubt whether abalone populations will ever rebound in Southern California, at least anytime soon.
To get an idea of the scale of abalones' decline: Between 1955 and 1970, commercial red abalone landings were stable at about 1,200 metric tons a year, according to data from the Department of Fish & Game. By 1995, only about 125 metric tons were landed. A year later every abalone species in the state had been fished or decimated by disease to the extent that the Department of Fish & Game in 1997 established a moratorium on all abalone diving south of San Francisco.
Withering disease, besides wiping out wild abalone beds, also poses a significant economic threat to aquaculture farms, which may rear the shellfish in land-based tanks or sea-based cages. At present, there are approximately 12 commercial abalone farms in California, with the bulk of production coming from four facilities. The wholesale price for "abalone in the shell" ranges from $18 to $25 per pound. Abalone farming is a $4 million to $5 million a year industry in the state.
To help reduce losses at abalone farms, California Sea Grant has funded Dr. Friedman to develop an antibiotic therapy that may cure withering syndrome. She has recently been developing ways to deliver the antibiotics orally, through abalone feed.
California Sea Grant is also funding Dr. Friedman and Dr. Ron Tjeerdema, also at UC Davis, to examine the sequence of events that trigger withering syndrome in red abalone. It is believed that warmer ocean water temperatures and food shortages associated with El Nino events may induce outbreaks of the disease. By understanding this mechanism and developing appropriate medications, it may be possible to significantly reduce abalone mortalities at aquaculture facilities.
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