Scientists have largely blamed the uptick on warming waters, which may make food more scarce in the usual foraging areas. But a study published Monday in Science presents another tragic factor: Sea lions are increasingly exposed to a neurotoxin that hurts their spatial memory, making them prone to getting confused and lost while searching for increasingly rare sources of food.
The neurotoxin isn't manmade — it's produced naturally by marine algae and amplified by tiny creatures that consume it — but we might still be to blame for the plight of sea lions, and perhaps for the deaths of countless other marine mammals. The algal blooms that produce this toxin, called domoic acid, flourish under the warmer waters brought about by climate change.
The effects of domoic acid don't come as a complete surprise. Scientists first started studying the compound's neurotoxic effects back in the 90s. Peter Cook, lead author of the study and now a postdoctoral researcher at Emory University, started studying domoic acid almost a decade ago as a graduate student at the University of California at Santa Cruz.
"One thing that had been known and fairly well demonstrated is that DA leads to fairly reliable neurologic conditions," Cook told The Post. While other neurotoxins might cause general brain damage, DA seemed to cause reliable, specific lesions on the hippocampus. In humans, this results in an illness called amnesiac shellfish poisoning when shellfish who've eaten the algae are ingested.
"It hadn't really been studied in marine mammals," Cook said. "People didn't see it as a large-scale ecological concern."
Cook and his colleagues worked with sea lions that had been found stranded and rescued by the Marine Mammal Center, testing their behavior and spatial memory for abnormalities and scanning their brains to look for signs of DA-related damage.
"We tested animals with nice clean healthy brains, ones with some damage, and ones with severe damage from the toxin," Cook said. "It tracked to a really high degree — these animals with really high damage were just not able to hold onto spatial information at all. That’s important, because there’s always the worry that the wild animal just doesn’t care about your stupid test.But we saw that the healthy guys learned really quickly and effectively." As the damage got worse, so did their performance on spatial memory tests.
Shawn Johnson, who serves as the Marine Mammal Center's director of veterinary science but wasn't involved in the study, explained that the rescue group has seen some bizarre things in recent years.
"The strandings we're seeing are just not normal," Johnson told The Post. "They end up stranding themselves in really abnormal locations, really out of their normal environments, in the middle of town, out in a field, and so on." In 2015, they've rescued 224 California sea lions with signs of domoic acid toxicity. "The study basically validates what we’ve known for quite a while from working with these sea lions," he said.
The center is working on developing treatments for these sea lions, Johnson said, but many of them are too impaired to be released back into the wild.
Most of the sea lions who are found stranded are underfed pups, and it's possible that even the ones with healthy brains can be linked to the neurotoxin — through their mothers.
"When you see an underfed but otherwise healthy pup, you assume something happened to the mother," Cook said. "We suspect that what happened is that the food was moving around, it was less predictable. And when you think of an animal with severe spatial impairment, that becomes a problem. These mothers might be dying out in the ocean."
Markus Horning, Science Director at the Alaska SeaLife Center, wasn't involved in the new study. But he thinks its findings are spot-on. "The study provides a convincing explanation of possible non-lethal effects of domic acid on the behavior, and secondarily on the foraging ability of sea lions," he told The Post.
While scientists already knew that the algae could cause seizures and death in the ocean, the new findings show how the toxin might also contribute to a general decline in sea lion success. "Altered behavior and significantly reduced foraging ability could result in increased strandings and mortality," Horning said. "The increased occurrence of harmful algal blooms in turn has been previously linked to changing ocean climate. Other potential causes include a reduced availability of suitable prey. This could in turn also be linked to altered ocean climate conditions. These two proposed causes could be creating synergistic effects."
Cook and his co-authors agree that human-driven climate change is quite likely to blame for the algal blooms, which are known to increase in size and frequency as waters grow warmer. "My impression is that it’s probably a major factor," Cook said. "It's complicated, and there are lots of factors. But you can trace a lot of these things back to warming oceans."
It gets worse: These algal blooms have also been suspects in a recent uptick in whale deaths. It's quite probable that whales and other marine mammals suffer the same confusion that sea lions do when impaired by the toxin, though further study is needed to make that connection.
"We often call sea lions the sentinels of the ocean," Cook said. "When they’re in distress, they come to shore. They’re visible. Most of the mammals in the same environment aren’t going to come in that way. A whale's instinct isn't to come to shore when it's feeling bad."