Harbor porpoises (Phocoena phocoena) had been missing from San Francisco Bay since Word War II, but they've returned in increasing numbers in recent years.
The marine mammals had been gone so long that the species may have collectively forgotten about the bay, Jonathan Stern, a cetacean researcher at San Francisco State University told NPR.
"Over 60 years, we're talking about a number of generations of porpoises," Stern said. "So it's quite likely that San Francisco Bay as a habitat was out of the institutional memory."
The habitat the porpoises fled during World War II was becoming a noisy, dirty and dangerous place for marine life.
During the war, San Francisco was a major ship-building center and naval yard. The Navy even stretched a 7-mile-long underwater net across the mouth of the bay to stop Japanese submarines from entering. Outside the bay, hundreds of mines made the waters treacherous.
After the war, things got worse. The postwar industrial and population boom flooded the water with sewage and factory pollution in the 1950s and '60s.
"I remember coming across the Bay Bridge when I was very young, and it would just smell," Bill Keener of Golden Gate Cetacean Research told NPR.
The Clean Water Act of 1972 signaled the beginning of the bay's rebirth. After decades of water quality improvement, the porpoises have returned.
Keener and his associates at Golden Gate Cetacean Research have identified 250 individual porpoises in the bay using distinctive scars on the animals' bodies.
"It's one of those very few good-news environmental stories. And it's in our backyard. It gives one hope," Stern said.
The harbor porpoise, Phocoena phocoena. (Credit: Erik Christensen, Wikimedia Commons).