Sea lion strandings linked to algal toxin: study

A study of California sea lions that stranded on shore said Monday an algal toxin disrupts their brain networks, leading to significant deficits in their spatial memory.

The study, published in the U.S. journal Science, may explain the impaired navigation observed in sea lions in recent years, which many believe has led to an increase in strandings.

Blooms of the toxic algae typically occur in the spring and fall along the California coast, but have been increasing in frequency and severity. Hundreds of sea lions strand on California beaches every year with symptoms of domoic acid poisoning, including disorientation and seizures.

A team led by Peter Cook, then a graduate student at the University of California Santa Cruz and now at Emory University, studied 30 California sea lions undergoing veterinary care and rehabilitation at the Marine Mammal Center in Sausalito.

They used magnetic resonance imaging to see the extent of brain lesions in the affected animals and found damage to the hippocampus, a part of the brain involved in memory, is often seen in sea lions with domoic acid poisoning.

"In this study, we were able to correlate the extent of hippocampal damage to specific behavioral impairments relevant to the animals' survival in the wild," he said.

The researchers then trained these sea lions on two spatial memory tasks, including one involving finding a food reward from among four potential locations.

Greater right hippocampal damage corresponded to poorer performance on the memory tasks, the researchers reported.

In addition to visible damage to the hippocampus, there were effects on interactions between the hippocampus and other brain structures, notably the thalamus.

"This is the first evidence of changes to brain networks in exposed sea lions, and suggests that these animals may be suffering a broad disruption of memory, not just spatial memory deficits," Cook said.

The study provides the missing link between the environmental degradation that leads to toxic algal blooms and sea lion strandings, said study coauthor Professor Charan Ranganath's Dynamic Memory Laboratory at the University of California Davis Center for Neuroscience.

"We didn't know exactly why the algae lead to strandings. But sea lions are dynamically foraging -- and for an animal like that, if you don't know where you are, you have a big problem," he said.

These memory deficits may also help explain anecdotal reports of sea lions showing up in unusual places far outside their normal range, either too far out at sea or inland far from the coast, said the researchers.

"What isn't well understood yet is the dose response," Cook said. "We don't know how heavy the exposure needs to be, or how often repeated, to cause this kind of brain damage, and we don't know the effects of repeated low-dose exposure." Enditem