Heart disease isn’t just the leading cause of death for humans in the United States. It’s also increasingly killing sea otters, especially adults in their prime — and now scientists know why.
Long-term exposure to domoic acid, which leaches from algae during toxic blooms commonly known as “red tides” and accumulates in sea otters’ favorite seafoods, is to blame, the researchers say. Algae blooms are becoming more frequent as climate change drives ocean temperatures up.
It’s long been known that domoic acid poisoning can cause neurologic disease in marine wildlife, like epilepsy or disorientation, often leading to death. Eating contaminated shellfish can do the same for humans, causing the severe illness amnesic shellfish poisoning that can result in permanent brain damage. But scientists are now concerned about the other health impacts of chronic exposure — to otters and people.
In a study published in January, researchers from UC Davis, the Monterey Bay Aquarium, the U.S. Geological Survey, the state Department of Fish and Wildlife, Department of Public Health, and UC Santa Cruz found that long-term domoic acid exposure increases the risk of fatal heart disease in sea otters off California’s Central Coast.
“Domoic acid exposure has more widespread health effects than we were aware of,” said Megan Moriarty, a wildlife veterinarian and lead author of the new study. “The biggest finding of this study pertains to sea otters, but it also goes on to include potentially other marine species, and even potentially humans.”
The study, published in the journal Harmful Algae, sheds light on an environmental toxin that’s threatening an important keystone species. Sea otters, which play a crucial role in keeping kelp forests and other areas near the coastline in ecological balance, are not only “sentinels for ecosystem health, but also for human health,” Moriarity said. That’s because they eat much of the same seafood as people do, like crabs, clams, and abalone.
When filter feeders like clams, mussels, and sardines eat the toxic Pseudo-nitzschia algae floating in the water, the domoic acid concentrates in their bodies. Crabs eat those filter feeders, concentrating it even more. By the time a sea otter dives and resurfaces with any one of those to munch on its belly, the domoic acid levels are significant.
Understanding the long-term impacts of domoic acid exposure will be important as algae blooms become more common and longer-lasting with rising ocean temperatures, she said.
The huge warm water “Blob” of 2015 in the Pacific Ocean, for example, caused toxic algae to bloom all along the West Coast. Domoic acid levels spiked so high that California had to close that season’s Dungeness crab fishery because of the danger of poisoning humans — which can manifest in abdominal cramps, vomiting, disorientation, and memory loss.
Moriarity said the research was inspired by another recent study of more than 15 years of data on the causes of death in southern sea otters. It found that shark bite was the most common direct cause of sea otter deaths during the study period of 1998 to 2012, making up 28% of the cases. When considering both primary and contributing causes of death, the most common culprit was infectious disease at 63%.
Cardiomyopathy, a disease of the heart, was the fatal blow for a shocking 41% of the otters.
“Cardiomyopathy starts off with some kind of inflammation in the heart muscle,” Moriarty said. “Over time, the heart becomes enlarged, and kind of dilated, and it doesn’t pump blood very effectively through the animal’s body. They can actually develop congestive heart failure similar to people. It can cause blood to back up into organs in the abdominal cavity and difficulty breathing. A lot of times, animals will be struggling for breath, or their lungs will appear wet, or they’ll have pneumonia.”
It’s a sad death for the lovely creatures.
To find out why this type of heart disease is so common in southern sea otters, which remain on the endangered species list in California, the researchers studied data from 186 southern sea otters from 2001 to 2017. After comparing the information to data on domoic acid levels, the scientists found that 34 of the 48 otter deaths during this time were caused by heart disease.
Otters that were repeatedly exposed to domoic acid over the years were 1.7 times more likely to die of heart disease than those in areas without algae blooms. For otters that ate mostly crabs and clams, which both accumulate domoic acid in their bodies and slowly excrete it into the water over time, the risk of death by heart disease increased by 2½ fold.
Even more disturbing was the surprising finding that prime-age adult otters are at greater risk of dying from cardiomyopathy than other age groups.
“If that age group is being hit hard by domoic acid and perhaps dying prematurely of heart disease,” Moriarity said, “then that really has some negative repercussions for future population growth.”
Southern sea otters are a threatened species. In the latest count, there were about 3,000 otters living along the coast in a range between Santa Barbara and San Mateo counties. Hunted nearly to extinction for their thick pelts in the 1800s, their recovery has been slow along California’s coast. Right now, their ranges end where great white shark activity picks up near the Santa Cruz-San Mateo County border, limiting otter populations’ ability to expand back into their historic range, which includes San Francisco Bay and areas farther north on California’s coast.
“That in and above itself is a challenge, and then on top of this, you have these different diseases that they get because they have to eat a lot” — 20-25% of their body weight each day — “to survive,” said Jessica Fujii, a sea otter researcher at Monterey Bay Aquarium and co-author of the paper. “And now we see that the food they’re eating could potentially increase the risk of some of these diseases.”
One of Moriarity’s big takeaways of this study was the benefit of long-term disease monitoring. Not only is it crucial for protecting wildlife, but it could reveal patterns in the environment, which could also help protect human health. “The value of these long-term monitoring datasets,” Moriarity found, “was just remarkable.”
There are several ways to limit the domoic acid problem, said Andrew Johnson, a sea otter research and conservation expert at Defenders of Wildlife. Curbing climate change in the years ahead, along with reducing polluted runoff from the land, including fertilizers from farms and lawns that contribute to algae blooms, can make a difference, he said.
“All that stuff gets in the watershed and creates harm down the line,” Johnson said.
For individual otters exposed to prolonged periods of domoic acid, “there’s probably not a whole lot that can be done,“ Johnson said. “But some of the things that we’re working on with the whole sea otter community, like reintroduction of sea otters into areas where they were historically and helping them expand beyond this little narrow range of California, will allow the population to both grow and be able to weather the effects of localized disease and domoic acid hotspots.”
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