The Collaborative International Dictionary of English v.0.48:

Blueback \Blue"back`\, n. (Zool.)
   (a) A trout (Salmo oquassa) inhabiting some of the lakes of
       Maine.
   (b) A salmon (Oncorhynchus nerka) of the Columbia River and
       northward.
   (c) An American river herring (Clupea [ae]stivalis),
       closely allied to the alewife.
       [1913 Webster]

WordNet (r) 3.0 (2006):

Oncorhynchus nerka
    n 1: small salmon with red flesh; found in rivers and
         tributaries of the northern Pacific and valued as food;
         adults die after spawning [syn: sockeye, sockeye
         salmon, red salmon, blueback salmon, Oncorhynchus
         nerka]