Short answers, pulled from the story.
The haddock's scientific name is Melanogrammus aeglefinus. Melanogrammus derives from the Greek words for "black" and "line", referring to the distinctive black lateral line running along the fish's body. The species name aeglefinus is a Latinisation of egrefin and eglefin, older vernacular names used in France and England.
Haddock is found on both sides of the North Atlantic but is more abundant in the eastern Atlantic. In the northeast it ranges from the Bay of Biscay north to Spitzbergen, with the largest stocks in the North Sea, off the Faroe Islands, off Iceland, and along the Norwegian coast. Off North America it extends from western Greenland south to Cape Hatteras, with the main commercial stock concentrated around Cape Cod and the Grand Banks of Newfoundland.
The Devil's thumbprint is a distinctive oval black blotch on the haddock's body, positioned between the lateral line and the pectoral fin. It is one of the most recognisable features of the species and gave the genus its name, Melanogrammus.
Finnan haddie is a smoked haddock product named after the Scottish fishing village of Finnan, also known as Findon. The fish was originally cold-smoked over smouldering peat. It is traditionally poached in milk and served for breakfast.
The commercial catch of haddock in North America ran to approximately 40,000 to 60,000 tonnes per year between 1920 and 1960, then collapsed sharply in the late 1960s to between 5,000 and 30,000 tonnes annually. Landings have not returned to historical levels despite some improved years after 1970. In the eastern Atlantic, landings fluctuated between 200,000 and 350,000 tonnes annually from 1980 to 2017.
The answer depends on the certifying body. Haddock currently sits on the Greenpeace seafood red list due to concerns about bottom trawling. The Monterey Bay Aquarium classifies it as a "good alternative". Many haddock fisheries hold Marine Stewardship Council sustainability certification, and all seven eastern Atlantic stocks assessed by ICES are considered to be harvested sustainably.