Abstracts
SEARCH Open Science Meeting
October 27, 2003
Seattle, Washington, USA
Linkages Between Climate, Growth, Competition at Sea, and Production of Sockeye Salmon Populations in Bristol Bay, Alaska, 1955-2000
Jennifer L. Nielsen1, Gregory T. Ruggerone2
1Alaska Science Center, USGS, 1011 East Tudor Road, Anchorage, AK, 99503, USA, Phone 907-786-3670, Fax 907-786-3636, jennifer_nielsen@usgs.gov
2Natural Resources Consultants, Inc., 1900 West Nickerson Street, Suite 207, Seattle, WA, 98119, USA, Phone 206-285-3480, Fax 206-283-8263, gruggerone@nrccorp.com
Bristol Bay, Alaska, supports one of the largest and most valuable salmon fisheries in the world. Salmon abundance in Bristol Bay and other northern areas more than doubled after the 1976/77 marine climate shift. However, in 1997/98, a major El Nino event lead to unusual oceanographic conditions and Bristol Bay sockeye salmon production was unexpectedly low. Nevertheless, the effect of climate on biological mechanisms leading to greater salmon survival and production are poorly understood. In order to test several hypotheses linking climate to salmon growth, interspecific and intraspecific competition, and salmon production, we measured annual marine and freshwater scale growth of Bristol Bay sockeye salmon, 1955 to 2000.
We discovered that the significant increase in sockeye salmon abundance during the late 1970s was associated with significantly greater salmon growth during the first and second years at sea, whereas growth during the third year was below average. Thus, the 1976/77 marine climate shift led to greater prey production, resulting in greater early marine growth and survival of sockeye salmon. Contrary to previous reports of density-dependent growth during early marine life, we found density-dependent growth was not readily apparent until the last year at sea when reduced growth typically has less affect on survival. In contrast with the 1976/77 climate shift, the 1997/98 El Nino led to significantly smaller size of adult sockeye salmon and lower survival, further supporting the hypothesis that growth at sea is strongly associated with climate and salmon survival.
Analysis of sockeye salmon scales also led to the discovery of significant interspecific competition between Asian pink salmon and Bristol Bay sockeye salmon in the North Pacific Ocean. The competition effect, whose detection was facilitated by the unique two-year cycle of pink salmon, led to reduced growth of sockeye salmon and a 35% reduction in survival at sea. Competition with pink salmon resulted in a loss of at least 59 million Bristol Bay sockeye salmon ($310 million ex-vessel value) during 1997-2000. This finding of competition provides the first clear evidence that interspecific competition at sea can lead to reduced growth and survival of salmon. Competition at sea has important new implications for stocks protected under the Endangered Species Act and for salmon hatcheries, which release more than four billion juvenile salmon into the North Pacific Ocean each year.
Abstract Categories: Biological Feedbacks
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