The Collaborative International Dictionary of English v.0.48:

Roundfish \Round"fish\, n. (Zool.)
      (a) Any ordinary market fish, exclusive of flounders,
          sole, halibut, and other flatfishes.
      (b) A lake whitefish (Coregonus quadrilateralis), less
          compressed than the common species. It is very
          abundant in British America and Alaska.
          [1913 Webster]