Regardless of where they are caught, they didn't spend their life there. Those fish come from the Lake Ontario, so they are the same fish that are caught in the Ganny or credit, generally speaking.
The old saying applies "The water is always dirtier in your own pond".
Well i suck as a philosopher, i do know that i have tried to eat them out of the trent and they taste NOTHING like salmon from BC... even though they are the same strain. I used to live in BC and as a kid watched my father laying the salmon on the picnic table before cleaning... meat went to the bbq after butterflying and guts went into the rose garden... but you can eat them after they spawn, some say they are mushy, but i haven't encountered a mushy salmon, and if i did i would probably make salmon cakes out of them
but they taste pretty good smoked, if you have a smoker, i personally wouldn't eat them after they turned green, or brown.