Green’s Dictionary of Slang

brown trout n.

[it ‘swims’ in the lavatory bowl]

excrement; thus brown trout pond n., a lavatory bowl.

[US]Bentley & Corbett Prison Sl. 89: Brown Trout [...] excrement.
[Ire]G. Coughlan Everyday Eng. and Sl. [Internet] Brown Trout (n): excrement.
[UK]Roger’s Profanisaurus in Viz Apr. 47: brown trout pond n. The lavatory bowl.

In phrases

fish for brown trout (v.)

(US gay) to have anal intercourse.

[US]R.O. Scott Gay Sl. Dict. [Internet] anal intercourse: [...] Syn: fish for brown trout.
sling trout (v.)

(US prison) to throw excrement or urine over another prisoner.

[US]Bentley & Corbett Prison Sl. 89: .Slingin’ Trout Throwing body waste on the guards passing the cells.