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Regime Scale Climate Forcing of Salmon Populations in the Northeast Pacific by R.C. Francis & S.R. Hare, University of Washington Idaho Farm Bureau News, May/June 1999

The objective of this paper is to discuss recent findings on the effects of regime-scale climate changes on upper ocean dynamics and apparent reposes in phytoplankton and zooplankton production in both the California and Alaska Current regions of the Northeast Pacific. These results have major implications concerning mechanisms linking the observe decadalscale climate response of Northeast Pacific salmon to their ocean environment.

Clearly much has been learned since 1982 (when Chelton first proposed his model) concerning the relation between atmosphere / ocean physics and Northeast Pacific salmon production. Based on the results summarized above, the following seems fairly clear:

1. The major climate influence on salmon production occurs at the decadal time scale, early in the marine life history, and in a bottom-up fashion through physical influences on primary and secondary production.
2. Plankton production seems to be influenced at the decadal (regime) scale by major climate-induced changes in the structure of the mixed layer. These influences appear to operate in opposite directions in the California Current and Alaska Current oceanic domains.
3. The farther south, the stronger is the influence of climate on biological production at the higher-frequency ENSO scale. For example, the coast-wide spike in Pacific Northwest salmon production which seemed to affect cohorts entering the ocean either in late 1984 or early 1985 could have resulted from a 1985 rebound to the 1982-83 El Nino (rapid cooling of the ocean and corresponding deepening of the mixed layer off California in 1985).
4. The effects of ocean circulation, particularly as they relate to the relative intensities of advection of subarctic water into the California and Alaska Currents, are less clear. Recent evidence points to a more complicated picture than the first speculated by Chelton (1984) at the first of these conferences in 1983. What does appear to be happening is that if advection and circulation are important, it is their effects on upper ocean structure (mixed layer depth, temperature) which, in turn, directly affects biological production.