I doubt they'd eat truly big salmon, but I have no problem imagining one of the big pikeminnows of yore swallowing up a big trout or a moderately sized salmon. They'd likely swipe at it in an ambush attack head first and use their pharyngeal teeth to subdue the struggling prey item until it is subdued enough to eat. I can imagine a hundred pound pikeminnow swallowing eight pound trouts and salmon whole and being able to handle up to about a 20 pound prey item.
It breaks my heart to see such a large, dynamic American predator going extinct. These are charismatic fish that need to be brought to the attention of the public in order to save them in a manner similar to what has been done with gar.
Even today, there is an
active eradication program for the northern pikeminnow due to their efficacy at eating trout and salmon.
edit- This is what they're killing off:
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