I really don't understand how a certain percentage of an endangered stock can be expendable. Perhaps alwaysfishn has a greater understanding of population statistics than myself
I'm not sure I can help you understand it any better. Understanding something like this requires looking at all sides of the situation.
From reading some of DFO's reports they determine what an acceptable by catch of an endangered stock is. For the upper Fraser coho I think their science determined that this run can survive a 5% hit.
While I am critical of some of DFO's management practices, they have the responsibility of protecting the wild salmon. They would have an easy job if they could just say no to the economic opportunities that a run like this presents, however the politics (and economics) says they can't do that. I see it the same as any environmental assessment for a new mine, a new development, a new pipeline etc. In most cases there is some detrimental effect to the environment. Saying no would be easy, but it would also shut down the economic opportunity which means jobs and taxes.