The regime concept and natural trends in the production of Pacific salmon

Large fluctuations in the trends of Pacific salmon production in this centu ry have been linked to trends in climate in the Pacific that are in turn as sociated with climate trends throughout the Northern Hemisphere. The close correspondence in the persistence of climate trends and the synchrony of th e changes is evidence that a common event may cause the regime shifts. The trends or regimes can be characterized by stable means in physical data ser ies or multiyear periods of linked recruitment patterns in fish populations . The regime concept is important in fisheries management because the natur al shifts in abundance may be large and sudden, requiring that these natura l impacts be distinguished from fishing effects. An equally important consi deration is that biological and physical mechanisms may change when regimes shift, resulting in conditions that may not be characterized in the earlie r part of the data series. Fluctuations in Pacific salmon abundance in this century were synchronous with large fluctuations in Japanese sardine abund ance, which can be traced back to the early 1600's. The synchrony in the fl uctuations suggests that Pacific salmon abundance may have fluctuated for c enturies in response to trends in climate. The concept of regimes and regim e shifts stresses the need to improve our understanding of the mechanisms t hat regulate the dynamics of fish and their ecosystems.