zoology.ubc.ca

Large-scale shifts occurred in climatic and oceanic conditions in1925,1947,1977,1989 and possibly1998. These shifts a¡ected the mix and abundance of suites of coexisting species during each period of relative environmental stability;from primary produ-cers to apex predators. However, the1989 regime shift was not a simple reversal of the 1977 shift. The regime shifts occurred abruptly and were neither random variations nor simple reversals to the previous conditions. Timing of these anomalous environ-mental events in the North Paci¢c Oceanappears to be linked to physical and biological responses in other oceanic regions of the world. Changes in the atmospheric pressure can alter wind patterns that a¡ect oceanic circulation and physical properties such as salinityand depth of the thermocline.This, in turn, a¡ects primaryand secondary pro-duction. Data from the North Paci¢c indicate that regime shifts can have opposite e¡ects on species living in di¡erent domains, or can a¡ect similar species living within a single domain in opposite ways. Climatic forcing appears to indirectly a¡ect ¢sh and marine mammal populations through changes in the distribution and abundance of their predators and prey. E¡ects of regime shifts on marine ecosystems are also mani-fested faster at lower trophic levels. Natural variability in the productivity of ¢sh stocks in associationwith regime shifts indicates that new approaches to managing ¢sheries should incorporate climatic as well as ¢sheries e¡ects. Correspondence: AndrewTrites