The Romans had a remarkably rich and varied diet. Fish played a key role in this diet. They were, above everything, a Mediterranean culture (they called the Mediterranean “Mare nostrum” or “Our sea”). Large fish, such as tuna, were cleaned, gutted and layered with salt in stone vats. This preserved them for longer periods of time and facilitated commerce throughout the Empire. Meanwhile, smaller fish were often used to make sauces.

Garum, liquamen, allec and muria all refer to Roman sauces created by crushing the whole fish and then fermenting it in brine. Pelagic fish, such as sardines, sprats, anchovies and mackerels were all used in various combinations. Garum was, by far, the most popular Roman sauce. It was so popular that it was produced in industrial quantities and traded in clay amphorae across the empire. Everyone knew it. “The overpriced guts of rotten fish,” grumbled the philosopher Seneca, who still kept it on his table. Meanwhile, Pliny the Elder called it an “exquisite liquid”.

Now, for the first time, researchers have found clear evidence of how garum was made — and sardines were the key element.

Stinky, Delicious, Fish Sauce

Garum was the “ketchup” of the Roman Empire. It flavored the meals of Roman emperors and peasants alike and was made everywhere from Spain to Greece and the Byzantium. The Romans made it in fish-salting plants, also known as cetariae, in coastal areas throughout the Roman Empire. Archaeologists have found several of these plants. The bottoms of fish-salting vats offer a myriad of remains, but it’s hard to analyze these remains. After all, the production process — crushing, salting, fermenting — utterly destroyed the fish, leaving behind only slivers of bone too small or broken to identify. Visual analysis, for decades, could only offer educated guesses.

Now, for the first time, researchers have sidestepped that problem by reading the DNA locked inside those ancient fish fragments. Led by Paula Campos of the University of Porto, the team extracted genetic material from a fish-salting vat at Adro Vello, a Roman site nestled along Spain’s Galician coast. What they discovered that the vat was full of European sardines.

The analysis isn’t straightforward. Fermentation creates acidic conditions hostile to DNA preservation, and garum production involved smashing up whole fish and leaving them to stew in brine. But thanks to meticulous techniques developed in ancient DNA labs (originally for studying Neanderthals and woolly mammoths researchers were able to piece together entire sardine genomes from the residue. It’s the first time anyone has successfully extracted and sequenced DNA from the bottom of a Roman fish-salting vat.

This sardine-based recipe fits with available descriptions of garum. The sauce tasted intensely salty, rich in umami, and slightly tangy — something between anchovy paste and soy sauce, with a pungent aroma that apparently appealed to Roman palates.

Recreating an Ancient Recipe (And Ecosystem)

What makes this find even more intriguing is that compared to modern populations, the Roman-era sardines were genetically purer, showing less mixing between fish populations. That suggests that in ancient times, sardines from different ocean regions didn’t mingle much. This is possibly because Roman fishing fleets worked locally, or because our centuries of industrial fishing and globalization have changed sardine populations. But this offers a type of baseline for understanding what some ecosystems were like during Roman times.

By matching ancient fish DNA to known species and populations, scientists can reconstruct what Romans ate, where they fished, and how far their trade networks stretched. The technique could even shed light on ancient fisheries’ sustainability, offering a rare, direct look at marine biodiversity before industrial exploitation began.

Of course, the finding also helps us better understand (and recreate) one of the most famous Roman sauces. And garum wasn’t just any food. As the Roman Empire spread, it brought with it roads, aqueducts, and fish sauces. From Europe to Africa and the Middle East, they established salting plants. Adro Vello, the site in Spain where this finding was made, was part of this vast network. There, the Romans not only produced garum but also likely exported it by sea in ceramic amphorae, fragments of which still litter the site.

Next up for Campos and her team: more vats, more sites, and more species. Other plants may have used anchovies, mackerel, and sprat. The team wants to know how recipes of garum varied across the Empire, mapping the different recipes.

Today, fermented fish-based sauces remain popular. We don’t have garum (at least not yet), but we have the classic Worcestershire sauce and the many fish sauces produced in Southeast Asia.

The study was published in the journal Antiquity.