Migrating summer chum and chinook salmon swim on July 7, 2007, below the U.S. Bureau of Land Managment's fish weir on the Tozitna River, Alaska. The Tozitna is a tributory of the Yukon River. (Photo by Jason Post/U.S. Bureau of Land Management) When 26-year-old Tirzah Bryant was a kid, she knew how to watch the cottonwood trees along the Yukon River for the sign that it was time to fish for salmon. “We know that the fish are coming when the cott…