Australian scientists say they've successfully restored a once thriving seaweed species that vanished during decades of high levels of sewage discharge.

The researchers transplanted fertile specimens of the missing crayweed, Phyllospora comosa, onto two barren reef sites along the coastline near Sydney where it had grown abundantly until the 1970s, the University of New South Wales reported Tuesday.

"Seaweeds are the 'trees' of the oceans, providing habitat structure, food and shelter for other marine organisms, such as crayfish and abalone," lead study author Alexandra Campbell of the university's Center for Marine Bio-Innovation said.

The researchers took seaweed from two reefs -- one north of Sydney and one to the south -- and transplanted it to Long Bay and Cape Banks near the city, where the seaweed had disappeared in the 1970s and 1980s, leaving "bald" reefs behind.

Despite improvements in ocean water quality around Sydney -- with improved sewage infrastructure -- a 40-mile stretch of underwater forest had never been able to recover naturally, they said, thus the transplant effort.

"The transplanted crayweed not only survived similarly to those in natural populations, but they also successfully reproduced," Campbell said. "This creates the potential for a self-sustaining population at a place where this species has been missing for decades."

The result has been "an environmental good news story," said research supervisor Peter Steinberg, director of the Sydney Institute of Marine Science.