The deaths of six sea lions found in traps on the Columbia River earlier this month were likely caused by the heat, and not by gunshots as officials first suspected, the National Marine Fisheries Service said.
Oregon and Washington officials had been trapping the animals as part of a federally approved removal process because they feast on salmon at the Bonneville Dam.
Federal and state officials initially said the sea lions had been shot, but they did an about-face after necropsies by state and federal experts found no evidence of bullet wounds.
The fisheries service said Wednesday the results of necropsies on all six animals were consistent with death from heat stroke. Studies of tissue samples taken after the May 4 deaths are expected in about 10 days and might reveal more.
The Humane Society of the United States questioned whether the 60-degree weather would kill the animals.
Marine mammal experts, however, said the blubbery animals are used to swimming in cold water and can rapidly overheat even when air temperatures are comfortable to humans.
Panic as the sea lions realized they were trapped in the cages could also have caused them to exert themselves, further elevating their body temperatures, Steven Brown, veterinarian for the Oregon Coast Aquarium in Newport, told The Oregonian newspaper.
At least one of the sea lions had lacerations believed to be inflicted by another animal.