If splitting the impacts equally means that a tribe cannot get at it's full, equal share of the harvestable fish, then how is that not affecting their ability to access their treaty reserved right and thereby putting a disproportionate conservation burden on them?

Then you say "Yes, but if they fished mark-selectively, they could get at their full share and not exceed 50% of the allowable impacts". Sure, but they don't have to.

Next time your fishing in the ocean or at buoy 10 and it's non-mark selective, just go ahead and throw all those unmarked fish back, even though you don't have to, it's easy!