This is for the UK:

https://www.mcsuk.org/goodfishguide/search

The app page has links to Android and iOS apps, and a PDF (although the latter is dense and highly coloured, not ideal to print).

This being for the UK, they also have a fish finger guide.

But what winds me up in the UK is how rarely i even have the option of eating sustainably. If i go to a fishmonger, or a decent-sized supermarket, sure. But in a small supermarket, or a pub or a fish and chip shop, it's always the standards: cod, haddock, skate, rock, plaice (and to be honest plaice is a pleasant surprise).

Of those, haddock is the only one that's really okay (scores 2-4): https://www.mcsuk.org/goodfishguide/search?name=Haddock

Cod is okay if it's from the far north, but most of the time you have no way of knowing (scores 1-5): https://www.mcsuk.org/goodfishguide/search?name=Cod%2C+Atlan...

Plaice is okay if it's from the east coast, but even then, the fishing methods are damaging (scores 2-5): https://www.mcsuk.org/goodfishguide/search?name=Plaice

Rock salmon is critically endangered (scores 5): https://www.mcsuk.org/goodfishguide/search?name=Spurdog%2C+S...

Skate is endangered and illegal to fish (scores 5): https://www.mcsuk.org/goodfishguide/search?name=Skate

Now, given that rock and skate are endangered, i have to admit i'm a bit confused about how they end up on menus. Are they being imported from other parts of the world where they're legal to fish? Or are they actually other, similar and legal, fish being sold under incorrect but familiar names?

The strange thing is I can remember when there was a push to use whiting over cod, but on that site the former is 3-5 while cod is 1-2

Breaded plaice is my favourite, although often chip shops don't seem to have any

seafoodwatch.org also has a list to other resources if you live in a country other than the USA: https://www.seafoodwatch.org/resources