Haddock

The haddock (Melanogrammus aeglefinus) is a saltwater fish found in the North Atlantic Ocean and associated seas.

The haddock is easily recognized by a black lateral line running along its white side (not to be confused with pollock which has the reverse, i.e., white line on black side) and a distinctive dark blotch above the pectoral fin, often described as a “thumbprint” or even the “Devil's thumbprint” or “St. Peter's mark”. (source)