n. — Fauna, Food
a commercially important saltwater fish that is found in northern seas, particularly the North Atlantic Ocean.
Type: 5. Frequency — The term is almost exclusively used in Canada (see Chart 1), which represents a late 20th-century development. As the 1917 quotation shows, at that time Boston bluefish appears not to have been harvested in Canada, a fact that changed in the late 1960s (see the quotations). The term appears to have originated as a nickname for a particular type of cod, specifically pollock. The species of fish is found in eastern North America, from New England to Nova Scotia, but can be found as far as Labrador and Greenland. According to Ross (1991: 158), Boston bluefish "reflects this species' appearance and fighting ability, not its culinary uses". Also used in the Northeastern US.
See also COD-2, s.v. "Boston bluefish", which is marked "Cdn", and DARE, s.v. "Boston bluefish" (n.), with New England quotations going back to 1946.
Images:
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Image 1: Pollock. Source: Wikimedia Commons. Photo: G. Jansoone 
Chart 1: Internet Domain Search, 8 Oct. 2013