Composed in 1819 when Schubert was 22 years old, one can hear the trout suddenly jumping out of the water and splashing at the very opening. But the name comes from the fourth movement, which is a set of variations on his song "Die Forelle" (The Trout). No autograph score exists - in a prodigious feat, Schubert appears to have directly written out the individual parts, from which the entire work was assembled and published posthumously in 1829.

Here's a rousing performance from the New England Conservatory.
https://youtu.be/7mq0JkRk25M