As a female entrepreneur in Vietnam's quickly growing economy, Nguyen Thi Tinh often travels around the country for work and leisure. She never leaves home without a bottle of her own fish sauce, brewed and bottled on the southern island of Phu Quoc. Regional variants of the pungent daily staple, called nuoc mam, leave her cold. 'Yuck! It's terrible. No way would I use it,' she grimaces. In her cool, shaded warehouse she draws a sample of her latest brew from a tall wooden vat. Showing it to a guest, she inhales deeply and dips her finger into the golden-brown liquid. The verdict? A sharp nose. Nice warm hues. And the taste is, well, sour, salty and unmistakably fishy. What cognac and champagne are to France, so the fermented fish sauce in Ms Tinh's vats is to Vietnam: a national treasure that shouldn't be produced anywhere else. And everyone agrees that the best fish sauce comes from the island of Phu Quoc. The islanders use only top-grade black anchovies, natural inputs and traditional storage methods to make their sauce, as they have done for a century or more. Wherever you travel in Vietnam, you're never too far from a bottle of fish sauce. It's a protein-rich mainstay of the cuisine and a constant companion to any savoury dish. Other Southeast Asian countries like Thailand and Cambodia produce their own sauces, but nobody consumes as much as the Vietnamese. 'Every morsel that people put in their mouths is either cooked in fish sauce or dipped in it,' says Ashok Mittal, vice-president of Unilever Vietnam's food division, which sells Knorr-branded fish sauce from Phu Quoc. Ms Tinh, 50, is a third-generation producer and president of Phu Quoc's fish-sauce association. Of six brothers and sisters, she was the one who decided to take over the family business. Since the economic reforms of the 1980s that opened up new markets for private businesses in Vietnam, she has added new capacity and upgraded her operation. She recalls her parents' stories of how the industry began, an almost accidental discovery of the sea's rich bounty. 'The anchovies were so plentiful, they swam right up to the beach. People would go fishing, but there were too many to eat, so they salted them and put them in pots. Then they found out that the extract tasted really good,' she says. Today, with over 80 fish-sauce producers on Phu Quoc, the industry is beginning to confront issues of sustainability. Fishermen are finding it harder to catch enough of the prized black anchovies in the waters around Phu Quoc and are forced to sail further offshore, often as far as 100km away, to haul in enough fish. They also use intense lights to attract anchovies to the boat, adding to their costs. A peculiar quirk of this part of the Gulf of Thailand is that the water isn't very salty, and so producers must import salt from elsewhere to preserve the fish. Boats carry jars of salt that are sprinkled onto the freshly caught fish as they dry in the sun on the journey back to shore. Pham Huynh Quoc inherited a medium-sized factory from his mother in the 1990s, but he is not upbeat about the industry's prospects. Last year the price of fish rose again because of supply problems, which he blames on the unregulated expansion on Phu Quoc. 'We have so many fish sauce manufacturers. I think in future it will be hard to find enough supply' of fish, he grumbles. But there's a new game in town: tourism. In recent years, as newly affluent Vietnamese take more vacations, beachfront property on Phu Quoc is being turned into resorts. The island is abuzz with rumours of foreign investors snapping up land and local officials are promoting Phu Quoc as the next big destination for holidaymakers in Southeast Asia. Mr Quoc runs a beach resort and is reinvesting his profits into the business, figuring that the returns are better than fish-sauce production. Ms Trinh is also dabbling in tourism with a small hotel, but she isn't forgetting her roots, or the product that makes this island famous among Vietnamese gastronomes. Inside her open-air warehouse where the sauce is prepared, dozens of tall wooden vats march along the concrete floor. The handmade vats are nearly 3 metres in diameter and can hold several tonnes of tiny, briny anchovies. For every three tonnes of fish, a tonne of sea salt is added, before the container is sealed at the top. After one year, producers unseal the vats and sample the first extract of this particular vintage. Just like the first pressing of olive oil, this extract is considered the finest, as it has the most intense aroma and flavour. Some is reserved for premium dipping sauce, while the rest is diluted with brine and allowed to ferment for longer. Traditionally this is women's work. 'Every housewife here knows how to make fish sauce. The husband would go out to fish and the women would stay home and make the sauce,' Ms Tinh explains. Until Vietnam began to tinker with its socialist economy in the 1980s, producers sold their sauce to the government at a fixed price. Private traders then took over. As demand increased, more families entered the business, chasing the profits that accrued to producers. Producers began to complain, though, that traders on the mainland were diluting their premium product with low-grade fish sauce and passing off the result as Phu Quoc sauce. Eventually, Vietnam's government took notice. In 2001, it ruled that only sauce produced and bottled on Phu Quoc could use the island's name, giving it the kind of territorial copyright that European wines and cheeses enjoy. At this point, Unilever, the Anglo-Dutch consumer-brand company, stepped in. It built a US$1 million bottling plant on Phu Quoc in 2002 and began selling Knorr-branded fish sauce in Vietnam. The move has proved very successful.