Just noticed this exchange. No, you can’t. Or rather, it depends.

Korean anchovy sauce is actually an active fermentation agent. In the old days, you pounded what were essentially slightly old and basically raw anchovies or certain kinds of squid (there are regional variations), and then you tossed this in with the vegetables. At a fairly cool temperature, in the dark, the combination of salt, hot chili powder, vegetable sugar, and the seafood would produce fermentation. Then, four months later or so, you dig up your pot of kimchi and it’s great. This is why kimchi keeps a long time and is tingly on the tongue. (Incidentally a variation on the same process creates sauerkraut.) Korean anchovy sauce is a modern derivative that is more stable and consistent than actual raw anchovies, and requires less careful handling. It does have a distinctive taste, but it’s not really “sauce” in that sense at all.

By contrast, Thai fish sauce is made from fermented fish, and the process is artificially stopped at the end. So it’s sort of like opposite ends of a process: anchovy sauce, once well fermented and so forth, turns into something like fish sauce. But fish sauce will not cause controlled fermentation, and it most definitely doesn’t taste the same.

That said, using fish sauce will produce a good flavor, albeit not a very Korean one. But you MUST NOT use it to enhance kimchi fermentation, or you are likely to end up with rotten kimchi — and I mean rotten as in rotting food, not as in “doesn’t taste nice.”

Good luck. Fortunately, if you’re into Korean food, you can mail-order anchovy sauce easily, and it keeps forever.

Can I use Thai/Vietnamese fish sauce instead?