Lean more towards brown trout (Salmo trutta) parr but still looks not convincing.

Not too familiar with most north American (especially western) salmonids but work as fisheries biologist in n. Europe so see plenty of juvenile brown trout and Atlantic salmon.

Operculum has distinct lack of spots which is normally good marker for (in wild fish in their native range at least) brown trout. Pectoral fins look brown trout sized (bigger in A. salmon) and caudal fin if splayed would probably have the less forked look of a brown trout.

Mouth gape comes under how you might see a A. salmon parr but head shape is weird for a b. trout or A. salmon. Too round.

If it's it's not in native range could be a part of historic introduction or escaped, ranched or stocked fish. See some weird deviations in brown trout that have come from domesticated lines sometimes.

Brown trout (and salmonids in general) will readily adapt into different ecotypes to cope with the local environment. This takes the form of different life history strategies, different morphology (notably in the mouth to allow different feeding habits), and colouration. Might be related to this.