Seems to be catchable in the West Bering Sea. Capture zone is indicated by the red circle on this map: sheema.blog.fc2.com/blog-date-20150823.html
A quick glance at the translated text suggests it's not easy to catch... or, is that the discovery version of this fish located in the same sea which is trickier to land?
Note: Apparently needed to produce Smoked Salmon, a North Sea cooking recipe (which also counts as an MA item).
Updated info: used as an ingredient in two recipes.
Smoked King Salmon (R13 Cooking) - found in the
North Sea Seafood cookbook sold at London's Item Shop
Ingredients:
Salt x1 +
Lumber x1 + King salmon x1
Yields "Smoked Salmon" x1; 70 Vigour-fatigue recovery, non-fleet food, MA item for Salmon Dishes MA.
Second recipe: located at San Francisco's tavern via the Barkeeper; requires R16 Cooking and an equipped 19th Century Adept of Ages title to make the recipe visible.
Ingredients:
Cheese x1 +
Bagel x1 + Smoked Salmon x1 (see above)
Yields x2 "Bagel Sandwiches", also a Salmon Dishes MA item. Pardon the quote marks, but I haven't made this dish myself, so I don't know GAMA's name for it.
Oh, and when Atlantis' final chapter, "3.5 - Americas" is released, the new private farm up by Alaska can yield King Salmon in bulk, like other farms do Tuna.
Note that you will need to have charted the Northeast Passage up to the Bering Strait to go after the King Salmon, or the farm where it can be grown.
Source: gvdb.mydns.jp/db/module/TradeDB/action/ItemShow?id=4910 (recipes)
gvo.gamedb.info/wiki/?Privatefarm (King Salmon farm info)