Nguyen Tran Xuyen, 37, in Phu Yen's Song Cau Town, is raising nearly 38,000 lobsters in 155 cages. Xuyen said that he usually earned a profit of VND3 billion per batch, which lasted between 10 and 12 months.

"Traders usually come to collect the lobsters when they reach between 0.5-0.7 kilos and mainly sell to our key market of China," Xuyen said. "However, China has recently refused to buy large-sized lobsters and turned to importing those of less than 0.3 kilos. This has caused the prices of large-size lobsters to fall sharply from VND1 million to just VND600,000 a kilo, while smaller lobsters have risen to VND800,000 per kilo."

Xuyen said that if this situation continues, he would lose nearly VND2 billion this season.

Song Cau Town is a lobster hub in Phu Yen with over 70,000 cages. Local farmers export hundreds of tonnes of lobster every year, with nearly 90 percent going to China.

According to local farmers, they're facing big losses after China suddenly changed to buying small lobsters this year.

Farmers in the neighbouring province of Khanh Hoa are also facing similar worries.

Pham Trung Kien said he has about 5,000 lobsters between 0.3 and 0.5 kilos each.

"Traders are paying between VND800,000-900,000 for each kilo of lobsters weighing below 0.3 kilo," Kien said. "Meanwhile, my larger size lobsters are paid only VND600,000 a kilo. I'm facing a loss of over VND200 million this crop."

In explaining the situation, Nguyen Thi Anh Thu from Thanh Nhon Seafood Trading Company said that due to economic difficulties, more Chinese consumers have turned to smaller lobsters.

"Large lobsters face a higher death rate than smaller ones during transportation," she continued. "They're also facing competition from low-priced US, Canadian, Pakistan, and Indian lobsters.

Thu, however, said that demand for larger-sized lobster in China was expected to change before the Tet Lunar New Year Holiday when people need them for bigger parties.

"We had a big order of over 10 tonnes of large-size lobster from our Chinese customer to be delivered in the final months of this year," she said.

Meanwhile, Le Thi Hang Nga, vice chairman of the Phu Yen Provincial Department of Fisheries, said that the Chinese market is usually unstable at the year's end, leading to losses for local farmers.

"We've warned farmers not to only depend on the Chinese market but to look for more markets and adjust farming plans to suit the situation," she said.