MONTEREY BAY, Calif — Tourists come here from around the world to watch whales. It’s common to see humpbacks leaping out of the water and fin whales slapping the waves with their flukes. If you’re lucky, you’ll catch a gray whale poking its head out of the water to scope out the surroundings. And if you’re really lucky, you’ll see a blue whale — at up to 165 tons, the largest animal on earth.

But all is not well here this year. Gray whales are dying in large numbers. Since January, at least 167 North Pacific gray whales have washed ashore dead from Mexico to Alaska. That’s probably just a fraction of the number that have actually died. Most will have sunk to the sea floor; scientists call these carcasses “whale falls.” But the number of known deaths is high enough that the National Oceanic and Atmospheric Administration has declared an “unusual mortality event” — a pronouncement that has sent scientists scrambling to figure out what’s going on.

Events like this are often the first warning sign that something may be seriously amiss below the waves. Particularly striking is that many of the whales washed ashore have been emaciated.

Despite the toll humans have exacted on them, gray whales, which can reach 49 feet in length and weigh 45 tons, are known for their curiosity about boats and friendliness toward people. Each spring, they migrate roughly 5,000 miles from their birthing grounds in the warm, shallow lagoons of Baja California in Mexico to their feeding grounds in the frigid waters of the Bering Sea off the Alaska coast, where they feast daily on up to 1.3 tons of mostly small crustaceans called amphipods. It’s crucial for these whales to eat enough to survive the grueling 10,000-mile round-trip migration, among the longest of any mammal.