A Seaweed 'Fish Sauce' You Can Make at Home

Fish sauce earns its reputation honestly: a bottle of fermented anchovies can carry an entire cuisine on its back. If you cook without fish, or you simply want the same deep, salty funk from the sea vegetable shelf, you can get remarkably close with kombu, dried shiitake, soy and patience. The result isn't a forgery — it won't fool a Hanoian grandmother and shouldn't try to — but it hits the same three notes fish sauce hits: salt, glutamate depth, and a quiet marine savouriness that makes dressings, stir-fries and dipping sauces taste finished.

Here's the two-stage method: a quick stove-top version you can make tonight, and a slow-lane fermented version for people who like projects.

What fish sauce actually does (so you can copy it)

Before substituting, it helps to know what you're substituting. Good fish sauce delivers four things:

1. Salt — roughly 20–25% by weight. It's a seasoning first.
2. Glutamates and nucleotides — free amino acids from months of protein breakdown. This is the umami payload.
3. Volatile funk — fermentation compounds that read as "ocean depth" and round out sharp flavours.
4. A hint of sweetness — from the fish itself, which balances the salt.

A seaweed version can't replicate item three exactly — that funk is specific to fermented fish protein — but kombu and shiitake cover glutamates and nucleotides abundantly, which is why this pairing powers the entirely fish-free kombu dashi tradition of Japanese temple cooking. Our explainer on the science of seaweed umami breaks down why glutamate plus inosinate-style nucleotides multiplies savouriness rather than adding it. And for the funk note, we cheat: a splash of fermented soybean liquid (the cloudy brine from a bottle of soy sauce, or a spoonful of miso) gives controlled, pleasant fermentation character. Our broader piece on fish flavours without fish maps the whole strategy.

The quick version (45 minutes, keeps months)

This makes about 350 ml — a standard bottle's worth.

Ingredients

• 20 g dried kombu (about two generous sheets)
• 15 g dried shiitake mushrooms
• 1 litre water
• 120 ml light naturally brewed soy sauce (or tamari)
• 1 tbsp dark brown sugar or palm sugar
• 1 tsp fine sea salt, plus more to taste
• Optional: 1 sheet toasted nori, torn, for extra marine depth

Method

1. Wipe the kombu with a dry cloth — don't wash it, the white surface powder is flavour, not dirt. Add it and the shiitakes to the water in a saucepan and soak for 30 minutes (overnight is better still).
2. Bring the pan to the barest simmer — small bubbles around the edge, never a rolling boil, which pulls bitterness from kelp. If you're adding nori, drop it in now. Hold 10 minutes, then remove the nori. Continue at a bare simmer 10 more minutes.
3. Remove from the heat. Fish out the kombu (save it — it can become kombu tsukudani) and the shiitakes (slice them for a stir-fry).
4. Add the soy sauce, sugar and salt while the liquid is warm. Stir to dissolve. Taste: it should be aggressively salty-savoury, noticeably sweeter than soy sauce alone, with a long finish. Adjust salt cautiously — remember this seasons food at teaspoon doses.
5. Cool, strain through a coffee filter or fine cloth for a clear bottle, and funnel into a sterilised bottle.

Because it contains water alongside the soy, keep the quick version refrigerated and use within about two months. Shake before pouring; a little sediment is normal.

The slow version: actually fermented (3–4 weeks)

The quick version is seasoned dashi; this version is a genuine ferment, and its flavour is meaningfully funkier. You'll need a starter culture — the lazy and reliable choice is a spoonful of live miso or a tablespoon of liquid from an unpasteurised jar of lacto-fermented vegetables, as covered in our fermenting seaweed guide.

1. Make the quick version above but skip the soy sauce and reduce the water to 750 ml. Cool completely.
2. Stir in 2 tbsp fine sea salt (roughly 3.5% of the liquid weight — enough to steer the ferment safely) and your starter.
3. Ferment in a clean jar, covered with a cloth secured with a rubber band, at room temperature, out of direct sun. Stir daily with a clean spoon.
4. After a week you should see small bubbles when stirred and smell a deeper, almost cheesy savouriness. Taste from day seven onward (clean spoon each time). When it tastes rounder, saltier-seeming and faintly funky — usually 2–4 weeks — it's done.
5. Strain, bottle, refrigerate. Fermented, it keeps six months or more; the salt content keeps it stable, and flavour continues to mature cold, like the real thing does.

If anything smells rotten rather than savoury, or grows coloured or fuzzy mould, discard and restart with more salt and a cleaner jar.

How to use seaweed fish sauce

Anywhere fish sauce goes, in roughly the same quantities, with one mental adjustment: it's slightly softer than a good anchovy sauce, so in very bold dishes you may use a spoonful more.

• Pad Thai and stir-fries — combine with tamarind, palm sugar and lime as usual; nobody at the table will clock the difference.
• Dipping sauces — with lime juice, sugar, chilli and water for a vegan nước chấm; superb over rice noodle bowls and summer rolls.
• Dressings — a teaspoon in a lime-and-oil dressing for green mango or cucumber salads.
• Fried rice, soups, braises — a dash at the end adds depth rather than fishiness. It also rescues bland tomato soups and lentil stews in a way that will surprise you.
• Mayonnaise and aioli — a few drops deepen a seaweed mayonnaise without any anchovy.

A word on sourcing: any food-grade dried kombu works, and farmed kelp from managed waters — the kind grown by the coastal farms Tidewright's Harvest programme works alongside — is a dependable, traceable choice. The kombu species profile explains what makes Saccharina japonica such a glutamate reservoir.

Iodine, briefly

Kombu is iodine-rich, and some carries into the sauce — but you use this by the teaspoon, not the cup, so exposure stays well within everyday ranges for most people. If you have a thyroid condition or are pregnant, read our sober rundown in iodine and seaweed and keep portions modest.

Key takeaways

• Fish sauce delivers salt, glutamates, nucleotides and funk; kombu + shiitake + soy + fermentation covers all four without fish.
• Quick version: simmer kombu and shiitake, season with soy, sugar and salt; refrigerate, use within two months.
• Fermented version: salt at ~3.5%, add a live starter, ferment 2–4 weeks for real funk; keeps six-plus months refrigerated.
• Use like fish sauce, possibly a spoonful more in bold dishes — it's slightly softer.
• Kombu is the glutamate engine; shiitake brings the nucleotides that make umami multiply.
• Never boil kombu hard — a bare simmer avoids bitterness.

FAQ

Does it actually taste like fish sauce?

No, and it shouldn't have to. It tastes like the savoury, salty, ocean-adjacent seasoning fish sauce acts as in a finished dish. Side by side you can tell them apart; in pad Thai or a dipping sauce, the functional role is identical and the difference is cosmetic.

Can I use wakame or nori instead of kombu?

Nori works as an accent — it adds oceanic top notes fast — but it can't carry the base; it's too thin in glutamates and disintegrates. Wakame contributes little depth. Kombu is the workhorse, as our wakame vs kombu vs nori guide explains.

Is tamari okay instead of soy sauce?

Yes — tamari gives a slightly richer, smoother result and keeps the sauce gluten-free. Avoid very sweet soy sauces (kecap manis) or heavily seasoned ones; you want a plain brewed base.

My ferment has a white film on top — is that bad?

A thin, cream-coloured, non-fuzzy film is usually kahm yeast — not dangerous, but scoop it off; it adds off-flavours at high levels. Coloured, fuzzy or smelly growth is a discard. Keeping solids submerged and salt at 3.5% prevents almost all of this.

Can I make it without mushrooms?

Yes — double the kombu and add a tablespoon of miso to the finished sauce. You'll lose some of the nucleotide synergy, but the sauce still seasons beautifully.