Bui Van Phong faced a choice when the Vietnam war ended 50 years ago: stay in his small village and help his parents carry on the family’s centuries-old tradition of making fish sauce, or join the hundreds of thousands of people fleeing his country for a better life.

Phong chose to stay behind and nurtured a business making the beloved condiment, known as nuoc mam in Vietnam, that is now in its fourth generation with his son, Bui Van Phu, 41, at the helm.

Fish sauce from the village has been recognised as an indelible part of the country’s heritage and the younger Bui is acutely aware of what that means.

“It isn’t just the quality of fish sauce. It is also the historical value,” he said.

But that heritage is under threat, and not only from giant conglomerates that mass-produce fish sauce in factories. Climate change and overfishing are making it harder to catch the anchovies essential to the condiment that underlies so much of Vietnam and Southeast Asia’s food.