Harbor porpoises began disappearing from San Francisco Bay during the height of Navy ship activity there during World War II. "We don't know why they disappeared. … It's very possible that they just abandoned the place because it became too hard to feed, reproduce and raise their young," said William Keener, a co-investigator and spokesman with the nonprofit Golden Gate Cetacean Research group. "Then all of a sudden, the porpoises were back."
"Local marine biologists began noticing the presence of the shy creatures inside the bay a few years ago, Keener said. By the fall of 2009, members of his group were regularly observing them underneath the Golden Gate Bridge and around the Angel Island area.
Scientists conjecture that bay water quality may have had something to do with the disappearance. If anything, it got profoundly worse in the decades after the war, but it's started to improve in recent years.
Others have theorized that global warming and rising sea temperatures are the reason for the shift."