garum was essentially the roman version of thai fish sauce or vitnamese Nunc mam (spelling?) - there were versions from both shellfish and fish, and also mixes. Not surprising - it's a concept that has re-occurred in several societies. and of course "rotted fish guts" would be more properly referred to as "fermented"

Which is why there is anchovy paste in several popular sauces of western culture - but they put it into teensy tiny print or list it as "additional natural flavours"