What north atlantic fish can we use to make a good ceviche?
I think just about any large flaky-fleshed fish that's not too fatty is a good candidate for ceviche. Salmon can work, though it's a tad fatty. Tuna is not a good choice, in my opinion. Cod and any kind of bass can work really well. I would think haddock or even sole or flounder could work too.

There is lots of white-fleshed fish available in the North Atlantic, and many will do great in ceviche. Take a look at the flesh and see if it seems similar to what you would usually use, and when in doubt ask the fishmonger for advice. Even if they don't know ceviche, if you can tell them what fish you'd usually use, they can probably advise you on substitutes.