PEBBLE BEACH, Calif.—First, the Dungeness crabs were poisoned. Then gray whales started turning up dead. Most recently, record numbers of Guadalupe fur seals and California sea lions are washing up starving and sick.

The West Coast this year has seen an unusually high number of marine animal deaths, sending scientists scrambling to understand why. The leading theories all have one factor in common: warming waters caused by climate change.

“These animals are really indicator species for ocean health, they’re the ones that are showing us right here from shore that there’s something wrong out there,” said Shawn Johnson, director of veterinary sciences at the Marine Mammal Center.