Green’s Dictionary of Slang

(US milit./prison) any species of fish served at mealtimes.

1919		K. Morse letter 14 Mar. in Uncensored Letters of a Canteen Girl (1920) 244: Cold ‘gold fish’ or ‘sewer carp,’ as the boys call their canned salmon, is just as bad as they say it is.
1935		E. Forbes Miss Marvel 130: One night Harold referred to the fried smelts as ‘sewer trout.’.
1941		S.F. Chronicle 1 June H5/7: The draftees assigned to Camp Claiborne, in Louisiana, got out a glossary of slang terms to describe everyday things in army life [...] they call canned milk an ‘armored cow,’ and the white fish a ‘sewer trout.’.
1944		P. Kendall Dict. Service Sl. n.p.: sewer trout . . . white fish.
1969		Ed Brandt Last Voyage of USS Pueblo 135: The men called it sewer trout and figured it was perhaps carp.
1981		Maledicta V:1+2 Summer + Winter 267: In the same manner, prisoners call any fish dinner in prison sewer trout.