What "Sustainably Sourced" Actually Means

"Sustainably sourced" gets used loosely on a lot of menus, but it has a specific meaning worth understanding, and a few related terms that get used almost interchangeably even though they mean genuinely different things.

The Formal Certification

In the UK, the most recognised sustainability mark is the Marine Stewardship Council's blue label, awarded to fisheries that meet a set standard for not overfishing a stock, minimising damage to the wider marine environment, and managing the fishery responsibly over time. A restaurant genuinely serving MSC-certified fish will usually say so directly, by name, not just describe it as sustainable in general terms. If a menu just says "sustainably sourced" with nothing more specific attached, that's a claim, not a certification.

Terms That Aren't a Formal Certification, But Still Mean Something

"Line-caught" means the fish was caught on an individual hook and line rather than a net, generally lower-impact and lower-bycatch than trawling, since it doesn't sweep up whatever else happens to be nearby.

"Day boat" fish comes from smaller boats that head out and return within a single day, generally fresher and from more local, smaller-scale fishing than a boat that stays at sea for weeks, since the catch reaches land, and often a kitchen, within hours rather than days.

"Wild-caught" simply means not farmed, it says nothing on its own about how sustainably it was caught, a wild-caught fish from an overfished stock is a real thing, worth knowing before assuming "wild" and "sustainable" mean the same claim.

The Farmed Equivalent of MSC

MSC covers wild fisheries specifically, it isn't the right mark to look for on farmed seafood, which is a genuinely large share of what's served in London (most salmon, for instance, is farmed rather than wild). The equivalent certification for farmed seafood is the Aquaculture Stewardship Council, ASC, a separate blue-and-green label covering responsible farming practices: feed sourcing, disease management, and impact on the surrounding water and wildlife. The two marks aren't interchangeable, and a menu correctly won't use MSC for a farmed item.

RSPCA Assured is a third mark worth knowing specifically for farmed salmon in the UK, a welfare-focused certification (stocking density, handling, water quality) rather than a stock-sustainability one like MSC or ASC. A product can carry more than one of these marks at once, they're checking different things, not competing versions of the same claim.

How Rare This Actually Is in London

Genuinely precise sustainability labelling is still rare on London menus specifically. In our own directory, only a handful of restaurants tag themselves with a specific sustainable seafood attribute rather than just using the word loosely in their own description, worth knowing if you're specifically looking for it rather than assuming most seafood restaurants already meet a formal standard. It's one of several signals worth checking alongside the freshness signs in our guide to spotting a genuinely fresh seafood restaurant, neither one alone tells you everything, but together they're a reasonable read on how seriously a kitchen takes sourcing.

Want to see how a specific restaurant describes its sourcing? Browse our full seafood restaurant directory across London.