REGIME SHIFTS

Regimes are multi-year periods of stable conditions in populations or physical oceanography. Concurrent and sometimes abrupt changes in these biological and physical variables indicate a regime shift. These natural patterns of variability are leading ecologists away from a single, steady state view of ecosystems to one that recognizes that ecosystems fluctuate because of internal and external sources of variability.

Indices of climate-ocean conditions indicate that a synchronous change occurred in the North Pacific in 1977. Since then, there has been a tendency for more frequent El Niño and fewer La Nina events. Such large-scale changes suggest that an as yet unidentified common, global event may be responsible for the shift.

The climatic shift observed in the late 1970s was not the first time such a change has occurred. In fact, it appears to have occurred at least three other times in the 20th century in 1925, 1947 and 1977. A fourth shift was reported in 1989, and there are indications that a fifth shift may have occurred around 1998.

The Aleutian Low Pressure Index from 1900 to 2001.
The 1925-46, 1947-76, 1977-88, 1989-97 and 1998- regimes are highlighted.
Note that the 1989 regime shift was not a simple reversal of the 1977 shift.

Ashleen Benson and Andrew Trites examined how the eastern North Pacific Ocean has responded to regime shifts (from primary producers through to the top predators in the food chain). Their review concludes that the timing of the anomalous environmental events in the North Pacific appear to be connected to physical and biological responses in other oceans. The available data suggests that the large-scale shifts that have occurred in the climate and oceanic conditions of the North Pacific were abrupt and were not random variations.

It appears that changes in atmospheric pressure alter wind patterns that affect oceanic circulation and physical properties such as salinity and depth of the thermocline. These in turn affect primary and secondary production, which in turn affects the higher trophic levels such as fish and marine mammals.

The available data suggest that regime shifts can have opposite effects on species living in different domains such as the Gulf of Alaska or waters of British Columbia and Washington, or they can affect similar species living within a single domain in opposite ways. Climatic forcing may also indirectly affect fish and marine mammal populations through changes in the distribution and abundance of predators and prey. Effects of regime shifts also appear to be manifested faster at lower trophic levels.

The growing body of scientific literature suggests that the productivity of fish stocks varies naturally in association with climate, and that the effects of fishing are superimposed on this natural regulation. This indicates that a new approach to managing the world's fisheries is required, one that incorporates climatic as well as fishing effects.

Full details and conclusions of the review are contained in Benson and Trites (2002).

1 Oct. 2002