I think that's related to the idea that we expect it will take time for the population to spread and increase in size. They will probably not mature until about five years of age. The generation time is an intermediate length of time compared to Great Lakes fish. We considered spread and the time it would take to increase the population size and felt that they would be closer to their maximum population size 50 years out than 20 years out.
On October 16th, 2012. See this statement in context.