A small North American freshwater fish (Semotilus corporalis) related to the carp, found in streams and rivers.
From 'fall' (waterfalls or rapidly flowing water where they're found) plus 'fish'. The name directly references the habitat where these fish are commonly observed.
Fallfish are basically the 'cockroach of North American streams'—they thrive in conditions where other fish can't survive, they're nearly impossible to eliminate, and yet fly fishermen love them because they're smart enough to make interesting catches despite being 'rough' fish.
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