The Collaborative International Dictionary of English v.0.48:

Sterlet \Ster"let\, n. [Russ. sterliade.] (Zool.)
   A small sturgeon (Acipenser ruthenus) found in the Caspian
   Sea and its rivers, and highly esteemed for its flavor. The
   finest caviar is made from its roe.
   [1913 Webster]