There is the idea that with additional marking to what already occurs—as Martin Paish pointed out earlier, it's only 10% of what's currently produced in hatcheries—if you moved to mass marking and also to an increase in mark-selective fisheries you are basically targeting hatchery-raised, hatchery-produced salmon. You are able to identify those fish very easily and distinguish between them and wild salmon.
Currently, where you have mixed fisheries, you allow hatchery retention, and the potential for increased wild harvest when you have that kind of fishery in play is greater than it is if you have periods of time in areas where you understand that the prevalence of marked fish is high enough. And if declare that this should be a mark-selective fishery, you can avoid wild salmon stocks, plain and simple.