Chinese fisherman Cai Chengzu claims he didn't know the 16-foot-long monster was endangered, and was planning to sell it at market.
While on a fishing trip Cai pulled in his net on Friday to discover a giant fish weighting two tonnes thrashing around, trying to escape.
The fisherman from Xianghzhi in the far-eastern province of Fujian, said that he and his colleagues were lifting the net out of the ocean when they realised that something had caused a huge rip in it.
In an interview with News 163 the fisherman said: "It's believed that the giant creature broke the net and got inside to eat the fish we caught."
Chengzu claims that the whale shark frantically to escape the net but died soon afterwards, and he decided to take it to the fishing harbour with the rest of his catch.
He reportedly planned to sell the huge fish for around 20,000 yuan (£2,000) at the wholesale market in Xiangzhi. However, his plan was thwarted when officials from the Fujian fisheries department halted the sale.
Whale sharks are a vulnerable species and classed as protected in China, making it illegal to either buy or sell them.
They are the largest of all fish species and an adult can live up to 100 years. They originated about 60 million years ago. They can grow to around 10 metres long, suggesting that, at 4.5 metres, the fish caught by Cai was a juvenile.
Whale shark populations may have been depleted by harpoon fisheries in Southeast Asia.
Because of their large size, whale sharks don't have many natural predators. There are only two reports of juvenile whale sharks being attacked by another creature – a Blue Marlin and a Blue Shark. Several whale sharks possess scars which are probably the result of shark attacks at an early age. Two killer whales have been filmed attacking, killing and consuming an 8 metre whale shark.
Whale sharks are highly prized on the black market. A single animal can be worth as much as £18,000, while the fins are sold to restaurants to make highly prized shark fin soup. The skin is also sold to manufacturers for bags, and other parts of the body are processed to make fish oil supplements.