Changing central Pacific El Niños reduce stability of North American salmon survival rates

Pacific salmon are a dominant component of the northeast Pacific ecosystem. In this new study, scientists from the Department of Wildlife, Fish and Conservation Biology, UC Davis, and Georgia Tech address the question of how recent changes in ocean conditions will affect populations of two salmon species (coho and Chinook). Since the 1980s, El Niño Southern Oscillation (ENSO) events have been more frequently associated with central tropical Pacific warming (CPW) rather than the canonical eastern Pacific warming ENSO (EPW). CPW is linked to the North Pacific Gyre Oscillation (NPGO), whereas EPW is linked to the Pacific Decadal Oscillation (PDO). Here the authors show that both coho and Chinook salmon survival rates along western North America indicate that the NPGO, rather than the PDO, explains salmon survival since the 1980s. The observed increase in NPGO variance in recent decades was accompanied by an increase in coherence of local survival rates of these two species, increasing salmon variability via the portfolio effect. The portfolio effect is an ecological phenomenon where increased biodiversity leads to increased ecological stability. Such increases in coherence among salmon stocks are usually attributed to controllable freshwater influences such as hatcheries and habitat degradation, but the unknown mechanism underlying the ocean climate effect identified here is not directly subject to management actions.