It was in fact a two-tonne 4.5m-long whale shark!
Chengzu says the whale shark tried to escape the net but died soon afterwards. He decided to take it to the fishing harbour with the rest of his catch, so he somehow lugged it up over his vehicle. He claims he didn't know it was endangered, and was planning to sell it at the wholesale market in Xiangzhi for around 20,000 yuan (NZ$4,000), until Fisheries halted the sale.
Whale sharks are a vulnerable species and protected in China, making it illegal to either buy or sell them. They're the largest of all fish species and an adult can live up to 100 years. They can grow to around 10m long, suggesting this one was a juvenile.
Whale shark populations have been depleted by harpoon fisheries in SE Asia. They're highly valued on the black market: a single animal can be worth as much as NZ$36,000, while the fins are sold to restaurants to make shark fin soup. The skin is also sold for handbags, and other parts of the body are processed to make fish oil supplements.