Six sea lions found dead in traps near a dam on the Columbia River in Oregon were killed by heatstroke, the National Marine Fisheries Service said.

Necropsies ruled out the possibility the animals were shot by vandals, The Portland Oregonian reported. Experts who examined the carcasses concluded the evidence was consistent with overheating.

Sea lions have a thick layer of blubber, which makes them vulnerable to overheating when they are out of water. In the cages, they would have been unable to cool off, and zoologists said stress could have hastened death.

State officials began trapping the sea lions at the Bonneville Dam to protect the salmon run on the Columbia. Sharon Young of the Humane Society of the United States said if the program continues the cages should be closely watched.

The expectation is that the animals are going to be safe there, but obviously they weren't, she said. Whether these animals were deliberately killed or they died of heat exhaustion, humans are responsible.

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