A Japanese salmon thought to have been extinct for 70 years has been discovered in a lake near Mount Fuji.

The kunimasu salmon, also called the black kokanee, is a subspecies of sockeye salmon that's found only in Japan. Unlike true sockeye, which migrate between freshwater and the oceans, the many types of kokanee salmon live and reproduce entirely in lakes.

The kunimasu was believed to have been wiped out in the 1940s after a hydroelectric dam raised acidity levels in the fish's only home, Lake Tazawako (map) in northern Japan's Akita Prefecture. Salmon are sensitive to water's acidity, and drastic changes in pH can affect young salmon's survival.

A seemingly unsuccessful 1935 program to release kunimasu eggs in Lake Saiko, in the foothills of Mount Fuji (map), had been forgotten—until recently, when the head of a local fishing association sent an odd sample to a Japanese television personality who is obsessed with fish.

Sakana-kun—a nickname meaning Mr. Fish—is best known for appearing on TV in his trademark white laboratory coat and blowfish-shaped hat. His self-taught expertise earned him the title of visiting associate professor at Tokyo University of Marine Science and Technology.

"The kunimasu salmon is very close in appearance to the common himemasu salmon, a species of landlocked sockeye salmon, except that one that was sent to me was green and black instead of the more common silver color" of the himemasu, Sakana-kun told National Geographic News.

Aware that the specimen was unusual, Sakana-kun sent the fish to Tetusji Nakabo, a professor of ichthyology at Kyoto University, where 17 samples of the extinct salmon had been preserved.

"When I first looked at it, I realized it might be a kunimasu," Nakabo said. "But it took me one month to confirm beyond a doubt my initial suspicions. At that moment, I felt it was incredible, unbelievable.

"Kunimasu is still alive!"

"Extinct" Salmon Not That Tasty

Further studies at Lake Saiko revealed four distinct characteristics that marked the kunimasu salmon apart from other close relatives in the fish world, Nakabo said.

The first indicator was that the kunimasu spawns in March, while the himemasu spawns in the fall. The newly rediscovered species also spawns at a depth of between 98 and 131 feet (30 and 40 meters), a behavior not seen in other species in Lake Saiko.

The final clues came from the number of gill rakers—bony, finger-like projections on the gill arches of filter feeders—and the structure of the pyloric caeca, finger-shaped stomach pouches that secrete digestive enzymes, Nakabo said.

The experts now think that some ten thousand kunimasu salmon inhabit Lake Saiko, which is fed by underground water from Mount Fuji, keeping its deepest reaches at the constantly cold temperatures the salmon prefer. (Also see "Salmon in the City: Fish Return to Paris River.")

People living near Lake Saiko had long caught the "extinct" fish, but the himemasu tasted better than the kunimasu, which was often thrown back, Sakana-kun said. The TV host added that he had wanted to taste the kunimasu that was sent to him, but he opted not to because it was the only one he had.

The Japanese government is now in the process of removing the kunimasu from its list of extinct species and drawing up plans to protect the fish's last known habitat.