Researchers trace toxins from algal blooms through the marine food web in Monterey Bay

January 09, 2001

Santa Cruz, CA -- Researchers studying a bloom of toxic algae in Monterey Bay last summer found the algal toxin domoic acid in anchovies, sardines, and krill, all key species in the marine food web. Harvesting of anchovies and sardines for human consumption was halted and there were no reports of adverse effects on wildlife from this particular bloom. Nevertheless, the findings raise concerns about the potential effects of the toxin on a wide range of marine mammals and birds, said Mary Silver, a professor of ocean sciences at the University of California, Santa Cruz.

Among the animals potentially affected by the toxin are several endangered species of whales that feed in Monterey Bay on the organisms Silver found were tainted with domoic acid. While there are no documented cases of whales dying from domoic acid poisoning, the toxin was blamed for the deaths of more than 50 California sea lions in 1998.

"We know the toxin enters the food web and poses a threat to large marine mammals and seabirds, but there is still a lot that we don't know," Silver said.

Silver has been studying domoic acid and the algae that produce it since the early 1990s. She and her coworkers presented their latest findings in December at a symposium in Woods Hole, MA, on harmful marine algae.

Domoic acid, a potentially lethal neurotoxin, is produced by several species of diatoms (microscopic algae) in the genus Pseudo-nitzschia. But not all blooms of these species are toxic, Silver said. During 2000, there were several small blooms of Pseudo-nitzschia that were not highly toxic, then a large one in late August and early September that had very high toxin levels. During this bloom, anchovies, sardines, and krill (all of which feed on diatoms and other kinds of plankton) accumulated enough domoic acid to be harmful to animals consuming them.

Krill are small, shrimp-like crustaceans that play a key role in marine food webs. A wide variety of marine organisms prey on krill in California's coastal waters, including seabirds, several species of endangered whales (humpback, fin, and blue whales), and commercially important species such as squid, sardines, rockfish, and salmon. Anchovies and sardines, in addition to their commercial value, are important in the diets of humpback whales, seabirds, seals, and sea lions.

Yet little is known about how different species are affected by consuming prey tainted with domoic acid, Silver said. The most well-documented cases of domoic acid poisoning in wildlife have involved seabirds and sea lions, which exhibit neurological symptoms, including seizures.

Silver said she is not sure why there were no reports of animals with symptoms of domoic acid poisoning during last summer's toxic bloom in Monterey Bay. More than 100 sea lions near San Luis Obispo showed signs of domoic acid poisoning during an unrelated bloom earlier in the summer.

Silver emphasized that monitoring programs are in place to ensure the safety of seafood for human consumption. In humans, domoic acid poisoning is also known as amnesic shellfish poisoning because it may cause permanent loss of short-term memory, in addition to other neurological and gastrointestinal disorders. In 1987, four people died of domoic acid poisoning in Canada after eating contaminated mussels, but such cases are rare. "If you obey the postings on the beach and don't harvest shellfish during a quarantine, you won't have any problems," Silver said.

Three other types of seafood toxins produced by marine algae are also known to occur along the California coast, according to Silver. The most recent addition to the list is the toxin okadaic acid, which causes diarrhetic shellfish poisoning. Silver's lab worked with an organic chemist in Japan to identify the toxin in algae from Monterey Bay last year. She plans to begin working with public health officials in the region to find out if people are being affected by this toxin.

"It will probably be hard to detect cases of diarrhetic shellfish poisoning, simply because there are so many possible causes of diarrhea," Silver noted.

Paralytic shellfish poisoning, caused by saxitoxins produced by various species of marine algae, was the first seafood poisoning syndrome recognized and linked with toxic algae. It was identified after an outbreak in San Francisco in the 1920s. It is a life-threatening syndrome, like domoic acid poisoning, but cases in humans are now rare due to effective monitoring programs.

There is also strong evidence for the occurrence in California of brevetoxin, which causes neurotoxic shellfish poisoning. Though not yet confirmed by direct detection of the toxin, brevetoxin is thought to have been responsible for the deaths of seabirds in Monterey Bay several years ago, Silver said.

The good news is that many scientists at UCSC and other marine research institutions around Monterey Bay are now studying these toxins, Silver said. For example, she has been working with researchers at the Monterey Bay Aquarium Research Institute who have developed new technology for monitoring domoic acid in the bay from instruments deployed on buoys.

"Harmful algal blooms probably occur in all coastal regions, but Monterey Bay is likely to become one of the best understood places in this regard due to the strong marine research community here. We can learn not only a lot about the toxins, but also about basic marine ecology and connections between organisms, while at the same time protecting human health, so it's very exciting," Silver said.
Editor's note: Reporters may contact Silver at 831-459-2908 or

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University of California - Santa Cruz

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