The Collaborative International Dictionary of English v.0.48:

Croaker \Croak"er\ (-?r), n.
   1. One who croaks, murmurs, grumbles, or complains
      unreasonably; one who habitually forebodes evil.
      [1913 Webster]

   2. (Zool.)
      (a) A small American fish (Micropogon undulatus), of the
          Atlantic coast.
      (a) An American fresh-water fish (Aplodinotus
          grunniens); -- called also drum.
      (c) The surf fish of California.
          [1913 Webster]

   Note: When caught these fishes make a croaking sound; whence
         the name, which is often corrupted into crocus.