Shifts in North Pacific ecosystems and their relationships to regional climate variability

Abstract

This study examines how the marine ecosystem along the western coast of North America has been affected by the pattern of the Pacific Decadal Oscillation (PDO) index and sea surface temperatures between 1950 and 2012. Previous studies on this topic have largely failed to incorporate the role of seabirds in the Pacific ecosystem. Data of commercial fish catch and seabird sightings were used for seven fish species and three bird species in California, Oregon, Washington and Alaska, as well as zooplankton biomass in California. Statistical tests were used to examine whether population sizes of these species changed in response to the potential phase shifts of the PDO index in 1977 and 1989. The results of this study indicate that the 1977 phase shift in the PDO index was actually a transition period from a ‘cool’ to a ‘warm’ phase that lasted approximately ten years from 1972 until the mid-1980s. This change was associated with marine ecosystem responses at all trophic levels. There is also some evidence for the presence of a second PDO phase change in 1989 and an associated ecosystem response. However, ecosystem changes at this time were less pronounced than those in the 1970s, probably because the PDO index was out of phase with ENSO variability. Responses of marine species to phase shifts in the PDO index were not synchronous across all U.S. states, with a particularly strong latitudinal pattern emerging for species of Pacific salmon. Furthermore, the results presented here indicate that localised sea surface temperature (SST) anomalies with the long-term global warming signal removed, may be able to account for more variation in population sizes than the PDO index, on the western coast of North America. These findings are important because they further our understanding of how species population sizes are affected by regional climate variability and they have the potential to inform decision making for fishing quotas and management plans in the region.