Northwest biologists are closely monitoring chum salmon this winter as they record what might be the largest run of Columbia River chum in 13 years.  The 2015 run could be as high as 20,000, making it the largest since 2002.   The annual run of Columbia River chum salmon historically numbered more than 1 million, but habitat loss and other factors caused their numbers to plummet during the last century to a low of just a few thousand per year.   The federal government listed Columbia River chum as threatened under the Endangered Species Act in 1999.  Chum are the last salmon of the year to return to the Columbia to spawn and the young are the first to leave for the ocean in the spring.  They generally spawn in the lower part of the Columbia River below Bonneville Dam.  The Bonneville Power Administration has funded two hatchery programs and construction of new spawning habitat for chum in several areas of the lower Columbia River.  In 2011, BPA increased the capacity of two chum salmon spawning channels located on tributaries just below Bonneville Dam.