A multi-agency scientific review panel, lead by scientists at the SWFSC Fisheries Ecology and Environmental Research Divisions and the NWFSC, released a report on March 18 that outlines the potential causes for the recent severe decline in the abundance of Sacramento River fall Chinook salmon (SRFC). The panel found that poor conditions in the coastal ocean in 2005 and 2006 resulted in unusually poor survival of fall-run chinook salmon returning to the river in 2007 and 2008. These fish entered the ocean during periods of weak upwelling, warm sea surface temperatures, and scarce food. Seabirds with diets similar to juvenile chinook abandoned their nests in these years because they could not feed their chicks.
Freshwater factors were ruled out as direct causes of the collapse because conditions were normal while juvenile chinook were in fresh water, normal numbers of juvenile fall-run chinook entered the estuary, and normal numbers of juvenile fall chinook were released from hatcheries into the bay. Water exports in the summer were high but did not occur until after the vast majority of juvenile fall chinook had already left the Delta. Freshwater factors are, however, implicated indirectly. Land and water development in the Sacramento-San Joaquin watershed has destroyed or degraded the once-diverse habitats that once supported highly diverse chinook populations. The remaining fall chinook populations lack the diversity that would make them more resilient to periods of poor ocean conditions. Most fall chinook are now born in hatcheries. Hatchery practices further reduce diversity and make the SRFC increasingly susceptible to periods of poor ocean conditions, but can create very large salmon runs when oceans conditions are good.
The panel noted that the key to reducing variation in the abundance of salmon is increasing their diversity. This could be achieved by reforming hatchery practices, managing for naturally-produced fall chinook, and restoring habitats in the rivers, Delta and bays.
Download the report:
Report on Sacramento River Fall Chinook Stock Collapse (PDF, 6MB)
Appendix (PDF, 3MB)
(March 18, 2009)