SAN JOSE, Calif. — Sitting 280 feet below water on the floor of the Pacific Ocean just 26 miles from the Golden Gate Bridge, a credit-card-sized underwater microphone represents the latest attempt to keep Earth’s largest mammals safe from human-caused destruction. The device, called a hydrophone, listens to the calls among blue, humpback and fin whales as they swim and feed off the Northern California coast. Every two hours, it reveals their identities and locations via data transmitted by a buoy on the surface. Back on shore, the locations of whale calls and sightings are superimposed over t…