China's river porpoises are rarer than pandas, but fishermen fighting to save them have been snared by a net of blackmail allegations, highlighting uncertainties faced by the country's emerging environmentalists.

Fewer than 1,000 finless porpoises -- grey dolphin-like animals with a hint of a grin on their bulbous faces -- are thought to remain in and around China's vast Yangtze river which carves through the centre of the country.

The porpoises are social and "pick up on human emotions like children would," said Hao Yujiang, researcher at China's Institute of Hydrobiology.

"They are the last mammals alive in the Yangtze, and they are a warning about the dire state of the river."

"The numbers have decreased very quickly," added Hao, who blames the decline on rampant overfishing -- sometimes with electric charges -- pollution from industry and sand-dredging ships trapping the animals in their propellers.

The government has pledged to protect those remaining, but environmentalists accuse local agencies of collusion with local industry.

"If the Yangtze porpoise is not delicious, why should we protect it?" state-broadcaster CCTV quoted a local official as saying last year.

Now the detentions of citizens seeking to protect the species are a demonstration of the difficulties facing grassroots environmentalists in China, whose numbers have swelled as citizens deal with an environmental hangover from decades of breakneck economic growth.

- A flash of grey -

With a bright orange life jacket and coat labelled "River Porpoise Protection Society" shielding him from driving rain, Liu Bo scans the vast waters of Dongting Lake where he patrols daily for the animals.

When a flash of grey appears between choppy waves, Liu's face lights up with excitement. "There's one!," he says pointing out from the edge of his small red vessel.

"When we see the porpoises, we feel like we've achieved something," he adds.

Liu, 34, was one of a band of fishermen who got together in 2010 to monitor the mammals and inform local authorities of others seen breaking fishing bans.

"Years ago, the porpoises would surround our boats and chase us like little children," Liu said.

"We want to make sure our grandchildren can see the porpoises. When I'm old I can proudly say I helped protect them."

But sailing has not been smooth. Two of Liu's colleagues -- He Daming and Li Jinsong -- were detained in March on charges of blackmailing local fishermen.

State-run media said that one had "confessed" threatening to report a local to authorities unless he was paid a 10,000 yuan fee.

Li remains in detention, while He is on the run from authorities -- leaving his wife Chen Yun distraught.

"He looked after the porpoises every day out of love, not for money," she said, holding a pair of his shoes, tattered from daily lake inspections.

Several local environmentalists told AFP that Li and He were framed by Xu Yaping -- a well-connected journalist from the local Communist party newspaper, who took over the patrol team in 2011.

Tensions had flared between Xu and He, and last year the latter established a breakaway porpoise protection group.

Xu's assistant, surnamed Wang, hung up the phone when called by AFP.

An associate who asked not to be named denied the accusations, adding that Xu is "well known man of culture and a fine calligrapher with excellent connections."

- Eased restrictions -

Though any groups seen to challenge the ruling Communist party are strictly banned, non-governmental or "civil-society," organisations covering less political areas have seen rapid growth in recent years.

Both Xu and He's groups benefited from an easing last year of restrictions on registration for non-governmental groups which saw 19,000 organisations register in 2013 alone, according to state-media.

"You can see that the local government supported the dolphin protection groups by allowing them to register, showing their trust," said Feng Yongfeng, a Beijing-based environmental commentator.

"The problems stem from internal issues between the groups, and Xu Yaping's connections have allowed him to make things worse," he added.

Several Chinese charities have been hit by scandal in recent years, stoking a lack of trust in the sector which analysts blame for low donations.

Just 10 percent of Chinese people donated money to charity last year, according to the US-based Charities Aid Foundation.

"If you look at the way civil-society has grown in the last decade, the growth has just been phenomenal," said Karla Simon, visiting professor at Harvard University and author of a book on Chinese non-governmental organisations (NGOs).

China lacks a national legal framework for charities, leaving them vulnerable to allegations of corruption, she added.

The arrests have made porpoise patroller Liu Bo cautious, and he refuses to call authorities using his own name when he spots illegal fishing on the lake.

"Our families tell us, don't go out on the lake, because you'll be arrested," he said, adding: "If people like us don't step up to protect the porpoise, no one else will."