Those gold fish have been there a few years now. Some are huge, up to 6 inches long. Thousands upon thousand of juveniles in the shallows/weeds.
They can't rotenone the lake. Too many people draw drinking water from it - among other issues like it being FFSBC's largest and most constant producer of Blackwater rainbows. As others have said - rotenone doesn't 100% work most of the time either.
Lastly, introducing a predator species, especially non-native would be bad, as this drains into the Quesnel and then Fraser. The Blackwater rainbows have really gotten on to the gold fish this past year. Many guys we talked to around the lake this spring were cleaning rainbows with full stomachs of them, and some fish were even puking them up during handling. A 2 inch long, hot orange woolly bugger or booby fly was killer in there this year. Saw/heard of more 10+ pound rainbows this year than I've seen or heard of in the last 3 years. Those Blackwater rainbows are definitely eating them, and it will only become more common as the Blackwater grow larger and key in on them more. I'd personally leave it go, and see what happens in the next couple years. From what I've seen, the majority of goldfish stay very shallow and don't compete with the rainbows for food. If anything, you're utilizing the food that was never once available in the 4 inch deep warm weedy water, and turning it into a more abundant, high energy food that the rainbows can see and key in on with ease. Only time will tell.