Bill Keener’s eyes widened as he peered through his binoculars and spotted the dark, shiny dorsal fins swiftly bobbing along the surface of San Francisco Bay. “There’s three of them coming right at us,” he said. It was a drizzly Tuesday morning in winter, and the Marine Mammal Center field researcher and I had been wandering along the pedestrian walkway on the Golden Gate Bridge for about an hour. Cars whooshed past us as we dodged bicyclists and paused at lookout points, keenly peering over the steel railing toward the murky turquoise water about 200 feet below. We were hoping to catch a glim…