15 Apr
David Beauchamp
Professor & Assistant Unit Leader-Fisheries, U.S. Geological Survey, Washington Cooperative Fish & Wildlife Research Unit, School of Aquatic and Fisheries Sciences, University of Washington
Pelagic Food Web Ecology in Puget Sound: Implications for Chinook Salmon
Abstract
Recent evidence of strong size-selective ocean survival and the importance of early rearing by ESA-listed Chinook salmon in the pelagic zone of Puget Sound (Duffy 2009) should sharpen our focus on processes that affect growth and mortality during this critical period. Growth can be affected by temperature and the quantity or quality of food. Over the last decade, inter-annual, seasonal, and regional differences in growth could be attributed largely to differences in feeding rate (a surrogate for food supply or access to supply). Much of the variability in pelagic feeding rate corresponded directly to the variable contribution of crab larvae, insects, and amphipods to the energy budget. The observed high, but variable feeding rate, coupled with strong size-selective mortality, suggests that competition could affect growth during pelagic rearing, and thus reduce ocean survival. The question becomes what is the relative importance of competition by hatchery and wild Chinook, other species of salmon, herring, or other forage fishes. Also, very little is known about the actual sources of marine mortality for salmon, but predation has long been suspected as a primary cause. Does much of this mortality happen during Puget Sound rearing? We will examine the current evidence for and potential magnitude of predation by resident salmonids on juvenile salmon and forage fishes in Puget Sound. Determining whether heavy mortality occurs while juvenile salmon are rearing in Puget Sound, or whether growth performance during this critical period simply influences mortality during subsequent life stages will inform managers and researchers about the role of the Puget Sound food web in supporting production and survival of Chinook and associated pelagic species.
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