This is where things get embarrassing: a former grad student in my lab had used those populations in a reciprocal transplant experiment years ago (with provincial permission, of course). A few cages were vandalized in Gosling Lake, releasing a few dozen fish into a huge resident population.
While the introduction was entirely unintentional, I comfort myself that this is just 1 of 1000's of lakes with stickleback. And, the accident now shows us the potential magnitude of immigrant fitness advantage, and the speed and genome-wide extent of conspecific introgression in its early years.
One lesson: avoid transplant experiments. These have been quite popular in stickleback over the years and so we were following in an established tradition, but I would now recommend against them.
A second lesson is the benefit of following focal populations for many years and being a hoarder when it comes to specimens and DNA.