The Romans were among the first societies to extensively exploit fish resources, establishing large-scale salting and preservation plants where small pelagic fish were fermented to produce sauces such as garum. Here, the authors demonstrate that, despite being crushed and exposed to acidic conditions, usable DNA can be recovered from ichthyological residues at the bottom of fish-salting vats. At third-century AD Adro Vello (O Grove), Galicia, they confirm the use of European sardines (Sardina pilchardus) and move beyond morphology to explore population range and admixture and reveal the potential of this overlooked archaeological resource.