Pacific Salmon & their Ecosystems

pp 245-275

The Role of Competition and Predation in the Decline of Pacific Salmon and Steelhead

  • Kurt L. Fresh

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In this paper, I examine the role of competition and predation in the decline of Pacific salmon and steelhead (Oncorhynchus spp.) populations along the Pacific coast of North America. Few studies have clearly established the role of competition and predation in anadromous population declines, especially in marine habitats. A major reason for the uncertainty in the available data is the complexity and dynamic nature of competition and predation; a small change in one variable (e.g., prey size) significantly changes outcomes of competition and predation. In addition, large data gaps exist in our understanding of these interactions. For instance, evaluating the impact of introduced fishes is impossible because we do not know which nonnative fishes occur in many salmon-producing watersheds. Most available information is circumstantial. While such information can identify where inter-or intraspecific relationships may occur, it does not test mechanisms explaining why observed relationships exist. Thus, competition and predation are usually one of several plausible hypotheses explaining observed results.

Competition and predation should not be considered primary causes of population declines. For competition and predation to contribute to anadromous population declines, something must occur to alter the outcomes of these interactions (e.g., predation mortality increases). Competition and predation are altered as a result of the following: introductions of nonnative, non-salmonid fishes, introductions of artificially produced salmonids, environmental changes, and non-environmental changes in predator or competitor populations (e.g., from fishing). Efforts to restore salmon populations must direct action at identifying and eliminating primary causes of population declines and not simply treating secondary effects (i.e., competition and predation) of these causes.