On October 23, volunteers counted 1,700 sea lions at San Francisco's famous Pier 39, but by November 21, that number dropped to 927. Seven days later there were 20, and now there are a scant half-dozen. Biologists don't understand why the sea lions left, but it's been a strange year for the barking mammals. A spike in birth rates in 2008 taxed local food supplies and led to record rates of starvation and injury among yearlings, but shot the number of adults on Pier 39 to record highs months later, before the speedy exodus began. The sea lions established themselves on the pier in 1990, when a large herring run seduced them into San Francisco Bay.