Photos of endangered hammerhead sharks on sale in a fish market in the Hainan resort city of Sanya have triggered an outcry among netizens.

The pictures were circulated on popular Chinese social media application WeChat on Saturday. Authorities in Sanya are investigating the incident.

Many netizen expressed their sadness and anger online. A netizen named Jiaxuan YoYo said she had spent several months' savings on a wildlife watching cruise just to get a glimpse of hammerhead sharks in the wild and was appalled to see them on sale at a fish market.

Many believed that the sharks were killed for their fins.

About 100 juvenile scalloped hammerhead sharks were being sold at a price of 30 yuan ($4.6) per kilogram in Sanya fish market on Saturday, local media Hinews reported.

Their fins are often used for dishes such as shark fin soup, an expensive delicacy in Chinese cuisine.

Scalloped hammerheads have been listed as endangered under the Convention on International Trade Species of Wild Fauna and Flora, to which China is a party.

However, weak awareness of these protections among fishermen means they still often catch hammerheads to harvest their fins.

Liang Yapai, a 65 year-old retired fisherman in Sanya, told China Daily that he used to sell the fins from sharks he had caught in the South China Sea for hundreds of yuan each.

"It was a lot of money at that time, but I never did it again once I found out the sharks were protected wild animals" he said.

Fellow fisherman Fu Cefa said he had only ever caught scalloped hammerheads twice, but admitted that he was not always sure which species he had caught.

Duan Deyu, deputy director of Sanya's bureau of ocean and fisheries, said the shark meat that was on sale in the market had been confiscated, one buyer had been identified and the incident was under investigation.

"We have asked experts to help us identify the meat, because it has been cut into several pieces," said Duan.

"We are still searching for the fisherman and the trader responsible. Fisherman, trader and buyer will be punished if it is confirmed to be shark meat."

In the meantime, fish market inspections will be stepped up and the government will do more to raise awareness of species protection among fishermen, the official said.

"We are now putting together a pamphlet with an NGO that will feature images and descriptions of endangered and protected wild species. The pamphlets will be distributed to fishermen as soon as possible.

And we will send guides to fishing boats to help improve fishermen's awareness of wildlife protection," said Duan.