A new study of California sealions poisoned by a deadly natural neurotoxin suggests that algal blooms might be a major cause of marine mammal standings. Exposure to high levels of domoic acid, produced by marine algae, is well known to cause seizures and death in marine mammals, as well as in humans who have ingested contaminated seafood.

Behavioural experiments led by Peter Cook of Emory University in Georgia, USA, have now shown that sealions that survive the initial acute illness are left with impaired memory and oraging abilities. Scans revealed lesions in the hippocampus, an area of the brain important for spatial memory and navigation. “The numbers of exposed sealions becoming stranded vary from year to year, but have been increasing over the past two decades,” Cook told BBC Wildlife. “The blooms producing domoic add are increasing in size, frequency and duration, likely due to warming oceans and direct human impacts such as agricultural run-off” he added. “More and more species maybe exposed to these toxins at harmful levels in the coming years.”

“It is really disturbing,” said Kathi Lefebvre of the US National Oceanic and Atmospheric Administration. “There is no reason to think that whale brains would not be impacted in a similar way to the sealions’ brains.” Lefebvre added that a link between domoic acid and whale standings has not been proven, largely due to the difficulty of obtaining fresh tissue samples for analysis. And, according to Cook, we currently have no way of measuring behavioural symptoms in wild cetaceans. “We do see the toxin in whales that die of many causes such as starvation and ship strikes, and we think that domoic add poisoning may impact an animal’s ability to forage and evade dangers,” Lefebvre added. “Further investigation is needed to confirm it, but Cook’s findings support this theory.”