They don't have any population numbers for anything. The numbers have been asked for and none were supplied. Their definition of pure strain is greater or equal to 99% and it's determined by pulling some fish and mapping their genome.
Heh, I could be such a $#!+ disturber and say "yeah, the Bow and Highwood". They presented it more as something they're looking at, but didn't say it actually will happen or where. The context around it was similar similar projects in the States that they've been watching. Find a pure strain population upstream from a hybridized population that has a barrier between them. The hybridized population should have a barrier below it as well to separate them from other hybrids. Nuke hybrids then transplant some pure strains into that stretch.