If you have forecast for X fish, and you're planning to harvest 20% of that X number at the dam, then why not require the passage of 80% of X *before* harvest begins?

In other words, why not harvest the LAST 20% instead of the first 20%?

Seems obvious, no? Then the upstream guys are guaranteed their fish, and if the estimate is low, the commercial guys don't rape the little run that materializes.