"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."

Problem is you are targeting the late returning fish, spread the "retention" out over the run. The retention could be based on the previous day or weeks return as a percentage of the previous 10 to 20 year average returns.