Massive blooms of toxic algae, also known as red tides, may have led to brain damage contributing to the stranding of thousands of sea lions on California beaches, researchers say.
The algal bloom that has spread from California waters all the way to northern Washington is releasing domoic acid, which can be toxic to humans and to marine mammals who consume concentrations of it in crabs, oysters, mussels, sardines and anchovies, they explain.
Sea lions exposed to the neurotoxin can suffer damage to their spatial memory, leading to them becoming confused and lost as they search for increasingly diminishing sources of food, the researchers report in the journal Science.
They conducted a series of tests on 30 sea lions that had been rescued after being stranded on state beaches, putting them through a simple maze to see if they could navigate it to find a food reward.
The animals exhibited a significant loss in spatial memory, the researchers found.
"The [study] results inform us of brain-behavior linkages that influence the survival of sea lions," said Colleen Reichmuth from the University of California, Santa Cruz's Long Marine Laboratory.
The massive blooms of algae are occurring in waters off the California coast that are as much as 6 degrees warmer than usual, the researchers say, and climate change is likely an important factor.
Changing climate and ocean conditions could create challenges for future generations of sea lions, says wildlife biologist Sharon Melin with the U.S. National Marine Fisheries Service.
"It is an ecosystem health issue, driven by great global processes" like climate change and other factors, says Melin, a specialist in sea lion health.
Although the situation with the algae and the sea lions is not directly linked to El Niño, the Pacific Ocean phenomenon "is not helping the situation," she said.
The first signs that California's sea lions were being affected by the algae toxin came in 1998 when hundreds washed ashore in Monterey Bay, and strandings of the marine mammals, often showing signs of confusion and seizures, have been seen almost every year since then.
Most of the recent strandings have been of malnourished sea lion pups, and even ones not directly affected by the domoic acid could be linked to the neurotoxin's effects on their mothers, says study lead author Peter Cook. He began his studies on the toxin while a graduate student at UC Santa Cruz and he is now a postdoctoral researcher at Emory University.
"When you see an underfed but otherwise healthy pup, you assume something happened to the mother," Cook says. "We suspect that what happened is that the food was moving around, it was less predictable.
In an animal with a severe impairment of spatial memory, that can be a significant problem, he says.
"These mothers might be dying out in the ocean," he suggests.