Tuna fish sauce
Korean seasoning sauce made from tuna or bonito extract and soy sauce, salty, smoky and full of umami, much milder than anchovy fish sauce.
Tuna fish sauce, chamchi-aek in Korean, is a liquid seasoning made from the extract of smoked, dried skipjack, a type of tuna, balanced with soy sauce, salt and usually a little sugar. It tastes rounded, salty and smoky-fishy, without the sharp, fermented smell of classic anchovy fish sauce, with which it should not be confused. That is why it is used for soups, egg dishes, vegetable side dishes and broths when depth is wanted but the fish note should stay in the background. It is sold in well-stocked Asian grocery stores, especially Korean ones, and by mail order.
Substitute:
The closest match is Japanese soy-based dashi concentrate such as shiro dashi, which is also built on bonito. At a pinch, mix light soy sauce with a little dashi or bonito flake stock; this matches the salt and umami but is less rounded. Thai or anchovy fish sauce is not an equivalent substitute: it is far stronger and more fermented in smell and should only be used in small amounts.