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I would guess tiger muskie too. There have been a few taken out of the reach.
I think a majority of them have been dumped by private stock or accidental release. Sort of like how bass always end up in my favorite trout pond. They are a sterile fish I believe so have to either be planted or escape. Mayfield used to have them, and may still be planted with them. Doesn't it hook to the cowlitz which flows to the columbia? No stretch in my mind that a fish cant somehow get downstream of the dam.