Who said they don't mingle? I don't recall anyone saying that.

What HAS been said over and over is that the resulting offspring do not return as adults in any significant numbers, and that's a problem because a viable wild parent has been removed from the pool by mingling with a hatchery fish.
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She was standin' alone over by the juke box, like she'd something to sell.
I said "baby, what's the goin' price?" She told me to go to hell.

Bon Scott - Shot Down in Flames