Harmful algal blooms are becoming more and more common in many areas of the ocean, including the Pacific. Certain types of algae can produce neurotoxins that accumulate up the food chain, finding their way into fish, and anything that eat fish, including sea lions.
Thousands of sea lions strand themselves on California’s beaches every year, and the number is increasing. This year, more pups came ashore than in the past decade combined; the animals come to the land malnourished, and sometimes too weak to swim.
A study to be published this week in the journal Science shows that many of these sea lions suffer from brain lesions caused by a toxin produced by a specific type of algae. The lesions form in the hippocampus, an area of the brain involved in memory—the stricken sea lions have impaired spatial memory and difficulty finding their way around.
In the study, the researchers conducted MRIs on 30 sea lions (which were undergoing treatment and rehabilitation at the Marine Mammal Center in Sausalito, Calif.) and found that those with lesions had trouble remembering where food was placed in an enclosure, compared to healthy sea lions; other tests showed indications of impaired short- and long-term memory. The findings help explain why sea lions are increasingly found stranded far from their typical range, suggesting that poisoning affected their spatial memory and caused them to get lost.
It’s the first paper to show the behavioral effects of long-term exposure to a toxin in a large marine mammal, and suggests that other large creatures like whales and sea otters may also be impacted in a similar way, says study first author Peter Cook, a researcher at Emory University who did much of the work on the study while finishing his Ph.D. in neuroscience at the University of California–Santa Cruz.
“There’s almost no doubt it’s affecting other animals as well, [including] whales, seals, otters—pretty much every marine mammal in the near-shore environment,” Cook says, considering the toxin has been measured in the blood of several other species.
In the wild, heavily affected animals would have more trouble finding food—in the worst cases, they’d be unable to find food at all, and likely die. The thing about sea lions is that they head to shore when in trouble, allowing them to rest and warm up in the sun—so we can see when one forgets where it’s going. But if other species were affected, we wouldn’t know, Cook says.
The study also “begins to explain why we often see increases in strandings long after a bloom has gone away,” says Raphael Kudela, a professor of ocean sciences at UC–Santa Cruz who wasn’t involved in the paper. It can take a while for the toxin to make its way up the food chain and accumulate in the bodies of the sea lions—little fish or crustaceans eat the algae; larger fish eat them; sea lions eat those larger fish.
The toxin in question is known as domoic acid, and is produced by a type of algae in the genus Pseudo-nitzschia. Domoic acid acts like the neurotransmitter glutamic acid, binding to and eventually damaging receptors in the hippocampus and causing short- and long-term memory loss.
The toxin has affected people before, most notably in a 1987 outbreak that started in contaminated Prince Edward Island shellfish, and killed three and sickened more than a hundred. Since that time, seafood has been more carefully and regularly tested for domoic acid, and fisheries may be shut down when it is found, Kudela says.
That actually has become a big problem for the seafood industry in California. This year alone there have been closures of anchovy, sardine, Dungeness, rock crab, mussel, clam and oyster fisheries in the state due to findings of contamination from domoic acid, Kudela notes. “This is starting to drive the smaller businesses out of the market, and we’re also losing millions of dollars in tourism for places like Washington State where recreational harvesting of razor clams is part of the culture,” he says.
The toxic blooms at the source of the problem are linked to increases in nutrient pollution, for example from fertilizer runoff, and warming water temperatures, both conditions that humans that exacerbated, Kudela says. The study suggests addressing these problems would help both animals and humans. “Sea lions are a sentinel species that tell us about the health of the oceans—this research shows that sea lions are responding to ocean conditions on both short and long timescales, and therefore provide us with an early warning for the natural and human-driven changes to our ocean,” Kudela says.