Heatstroke killed six trapped sea lions at Bonneville Dam

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Necropsies - With the cage doors shut, the animals couldn't reach the water to cool off

Six sea lions found dead below Bonneville Dam about a week and a half ago apparently died when they overheated inside floating cages set out by state officials on the Columbia River, federal authorities said Wednesday.

Investigators looking into the deaths said they still do not know whether the doors of the cages closed inadvertently or someone yanked on ropes to shut them.

However, the disclosure of how the animals died raises the possibility that the federally protected sea lions were killed not by foul play, but by a state trapping program gone awry.

Marine mammal experts said sea lions used to swimming in cold water could quickly overheat even when air temperatures are not very high. The animals' thick blubber, which insulates them from the cold ocean, could also act as an uncomfortable blanket when it's warm.

Panic as the sea lions realized they were trapped could also have caused them to exert themselves, further elevating their body temperatures, said Steven Brown, veterinarian for the Oregon Coast Aquarium in Newport. He compared it to a person wearing a double wetsuit and then strenuously exercising.

At least one of the sea lions had lacerations believed to be inflicted by another animal, a hint of tension inside the cages.

Federal and state officials first blamed the sea lion deaths at Bonneville on vigilantes, concluding the sea lions had apparently been shot. But they backtracked after necropsies by state and federal experts found no evidence to support that

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