A captain and three sailors from Quang Ngai Province were killed Saturday after falling into the fish hold, which police suspect contained methane, local news website Vnexpress said Monday.
Fisherman Nguyen Cong Tho slipped first and captain Nguyen Van Thang with two other sailors - Nguyen Cong Binh and Le Minh Phung - climbed down to help. They all died.
Another sailor Nguyen Minh Ngoc tied a string to his body before climbing down, only to be pulled up unconscious.
The fishermen, 18 to 34 years of age, were sailing with five others to Da Nang Port.
Investigators said the fishermen were killed by poisonous gas in the hold, possibly methane.
According to experts, stuffy space used after a long time without cleaning usually accumulate a large amount of methane gas, which kills people quickly.
Le Van Mui, chairman of Duc Pho District, Quang Ngai where the fishermen came from, said provincial authorities are investigating the case further towards prevent similar incidents in the future.
Mui said the deceased fishermen were all poor, so the district would find ways to help their families make a stable living.