I'm not really sure that there is a production obligation in the Mitchell Act, but rather an expectation of some hatchery obligation that has developed over the years. The entire act is only four paragraphs long and its purpose is "To provide for the conservation of the fishery resources of the Columbia River, establishment, operation, and maintenance of one or more stations in Oregon, Washington, and Idaho, and for the conduct of necessary investigations, surveys, stream improvements, and stocking operations for these purposes." The Mitchell Act
A 2010 view of the meaning of the act could easily deal with acts to preserve natural populations to conserve the fishery resources rather than the view of the 1930's and 40's that some number of millions of hatchery fish needed to be produced to conserve the fishery resource. After all, it is the status of the natural populations that actually controls access in any fishery, read 15% allowable impact versus something higher if the populations were in better shape.