"The exposed mice just did not learn," she said.

But after a nine-week recovery period, the mice bounced back and were able to work their way through the maze as well as mice that had never been exposed, she said. There was no sign of brain-cell death for the exposed mice, either, she said.

"So this is not a permanent issue," she said. While "clinical" levels of toxin exposure cause seizures, brain damage and sometimes death, the damages from sub-clinical exposures are "reversible as long as you don't slip into that clinical seizure effect," she said.

Still, even temporary toxin-caused impairments could make marine mammals vulnerable, Lefebvre noted. Questions remain about possibly compromised navigational skills, lowered defenses against predators and reduced ability to avoid collisions with vessels — all issues that would have to be considered in the future.