The Collaborative International Dictionary of English v.0.48:

Birt \Birt\ (b[~e]rt), n. [OE. byrte; cf. F. bertonneau. Cf.
   Bret, Burt.] (Zool.)
   A fish of the turbot kind; the brill. [Written also burt,
   bret, or brut.] [Prov. Eng.]
   [1913 Webster]