Pacific Salmon in Aquatic and Terrestrial Ecosystems

almon runs in the Pacific Northwest have been declining for decades, so much so that many runs are threatened or endangered; others have been completely extirpated (Nehlsen et al. 1991). This “salmon crisis” looms large in the public eye, because it has serious and wideranging economic, cultural, and ecological repercussions. Billions of dollars have gone into industrial and agricultural projects that alter regional rivers in ways that, often unintentionally, make them inaccessible or unsuitable for salmon. Recently, billions more have been spent in largely unsuccessful attempts to restore the languishing salmon runs (Lichatowich 1999). Moreover, enormous nonmonetary resources have been expended in assigning and denying responsibility for failed runs and debating the possible efficacy of various remedies. As resources that are devoted to reversing declining runs of salmon have increased, scientists and resource managers have been expanding our understanding of the ecological role of salmon and other anadromous fishes, which return from the sea to spawn in fresh water. We have known for years that spawning salmon serve as a food resource for wildlife species (e.g., Shuman 1950) and, when they die after spawning (as most Pacific salmon do), their carcasses provide nutrients (e.g., carbon [C], nitrogen [N], phosphorus [P]) to freshwater systems (e.g., Juday et al.1932). More recently, scientists have documented that these “salmon-derived nutrient” subsidies may have significant impacts on both freshwater and riparian communities and on the life histories of organisms that live there (Willson et al. 1998, Cederholm et al. 1999). Because of the burgeoning interest in salmon, growing indications of their ecological importance, and recent calls for management to consider the role of salmon in aquatic and terrestrial ecosystems (e.g., Larkin and Slaney 1997), we take this opportunity to review what is understood about the function of salmon as key elements of ecological systems. Our objectives are twofold. First , we expand on previous reviews of salmon (Willson et al. 1998, Cederholm et al. 1999) to include recent research that has amplified and modified earlier ideas about the contribution of salmon to ecosystem processes. In doing so, we describe the composition, magnitude, and distribution of marine inputs to freshwater and terrestrial systems via salmon. We use an expanding group of studies pertaining to stream nutrient budgets and salmon physiology to construct a schematic that illustrates salmon-derived products and the pathways by which they enter and are retained in aquatic and terrestrial food webs. We then consider the ecological variation associated with salmonid ecosystems and how this may influence the ecological response to the salmon input. Second, we consider how this variation in ecosystem response may influence management and conservation efforts.