Poor oceanic feeding conditions and commercial fishing have been identified as key causes of the collapse of Sacramento River fall run Chinook salmon that began in 2007. According to the National Marine Fisheries Service, immediately prior to the sharp salmon decline in 2007, rising temperatures in the Pacific Ocean depleted much needed nutrients. Salmon that spend most of their life in ocean waters were decimated by the lack of food available as a result of changing ocean conditions. Those oceanic conditions have dramatically improved over the past two years, with returning cold water flows supporting much needed nutrients for salmon species, and salmon runs in the Delta are beginning to improve as a result.
The Pacific Fisheries Management Council estimated that approximately 50 percent of adult salmon were being harvested by commercial fisherman. This dramatically reduces the number of salmon returning upriver to spawn and increased the prevalence of hatchery born salmon. In comparison, a study by the Department of Water Resources found that only 0.4 percent of Delta salmon are lost directly at the Delta pumps.
Fishery experts forecast banner salmon season Article from the San Francisco Chronicle (March 8, 2012)