I made the mistake of claiming on twitter to know everything there was to know about Ancient Greek fish sauce. I guess that's why I'm a twit. So by popular demand (that's you, Mr Cameron) here is my short article on garos, garum, and Worcestershire sauce.
As with many things, first came the Greeks. They had a salty fish sauce called garos (γαροσ). Since there was a fish called garos, or garon, in Greek, it's a fair bet the sauce was made mostly from that. I've been unable to discover what fish garos actually was. Not to worry, I can fudge it in my stories (but don't let Janet know that...I'll tell her I'm using the Greek word for authentic atmosphere).
The earliest references of which I'm aware are some lines in Aeschylus (fragments of the lost play Proteus) and Sophocles (fragments of the lost play Triptolemos), both of which refer to garos as stinking. Not a great advertisement, but the sauce was obviously popular enough that writers were referring to it and expecting everyone to understand. Since they were writing at the same time Nicolaos and Diotima are solving murders, I know I'm on solid ground using the sauce.
The stink is understandable. Although later Roman garum was made from carefully chosen gourmet fish, the original Greek version was made from leftover entrails.
Gary's theory, for what it's worth, is this: over-population was chronic in classical Greece, and children, especially small girls who were last in the feeding line, regularly went to bed hungry. Nothing that was even remotely edible was ever wasted. So when fishwives gutted the morning catch, they would have discarded the entrails into the large vats where some extra seawater would have been added, and the whole goopy mess allowed to ferment in the sun over weeks or months into garos. If this theory is correct then garos-the-fish is going to be whatever the main catch was.
When the Romans picked up the sauce from the Greeks, the ingredients and the name changed slightly. Garum isn't Latin. It's latinized Greek. Garum was made from whole fish, not only the offal. The Romans got very precious about the whole thing and would debate which species made the best sauce. Martial even talks about making it from, "mackerel still breathing its last." Later on, humble garum split into a range of gourmet sauces, each with their own names. Liquamen appears to be the original garum, and there was also allec, muria, and a pile of others. I haven't chased down any of these because by then, my characters are all shades in Hades.
Under Roman law it was illegal to make garum at home, the stench was that bad, so they had garum factories by the coast. The Greeks had no such rule, but practicality indicates garos would not have been made in Athens anyway, but Piraeus, the port town down the road, where the fisherman brought in their catch and the fishwives processed the fish. The garos would have been transported up to Athens in amphorae and sold in the Agora.
Jonathon raised the fascinating point that Worcestershire sauce is said to be a descendant of garum. Wikipedia disagrees, saying Worcestershire is derived from India. I'd normally be more inclined to believe Jonathon, given my record of finding errors in Wikipedia, but a large number of sites report the origin of Worcestershire as an attempt to reproduce an Indian sauce that went horribly wrong, and when two years later the failed experiment was sampled before discarding, there was Worcestershire sauce.
There is no chance that Worcestershire sauce tastes like garos, because the Greeks are known to have disliked anchovies. Also Worcestershire includes (apparently) molasses, chilies and sugar, none of which the Greeks had.