The Collaborative International Dictionary of English v.0.48:

Grayling \Gray"ling\, n. [From Gray, a.]
   1. (Zool.) A European fish (Thymallus vulgaris), allied to
      the trout, but having a very broad dorsal fin; -- called
      also umber. It inhabits cold mountain streams, and is
      valued as a game fish.
      [1913 Webster]

            And here and there a lusty trout,
            And here and there a grayling.        --Tennyson.
      [1913 Webster]

   2. (Zool.) An American fish of the genus Thymallus, having
      similar habits to the above; one species (T.
      Ontariensis), inhabits several streams in Michigan;
      another (T. montanus), is found in the Yellowstone
      region.
      [1913 Webster]