Division of Comparative Physiology and Biochemistry, Society for Integrative and Comparative Biology Routine and Active Metabolic Rates of Migrating Adult Wild Sockeye Salmon ( Oncorhynchus nerka Walbaum) in Seawater and Freshwater

We present the first data on the differences in routine and active metabolic rates for sexually maturing migratory adult sockeye salmon (Oncorhynchus nerka) that were intercepted in the ocean and then held in either seawater or freshwater. Routine and active oxygen uptake rates (Mo2) were significantly higher (27%–72%) in seawater than in freshwater at all swimming speeds except those approaching critical swimming speed. During a 45-min recovery period, the declining postexercise oxygen uptake remained 58%–73% higher in seawater than in freshwater. When fish performed a second swim test, active metabolic rates again remained 28%–81% higher for fish in seawater except at the critical swimming speed. Despite their differences in metabolic rates, fish in both seawater and freshwater could repeat the swim test and reach a similar maximum oxygen uptake and critical swimming speed as in the first swim test, even without restoring routine metabolic rate between swim * Corresponding author. Present address: Marine Sciences Research Center, 145 Endeavour Hall, Stony Brook University, State University of New York, Stony Brook, New York 11794-5000; e-mail: [email protected]. Physiological and Biochemical Zoology 79(1):100–108. 2006. 2006 by The University of Chicago. All rights reserved. 1522-2152/2006/7901-4098$15.00 tests. Thus, elevated Mo2 related to either being in seawater as opposed to freshwater or not being fully recovered from previous exhaustive exercise did not present itself as a metabolic loading that limited either critical swimming performance or maximum Mo2. The basis for the difference in metabolic rates of migratory sockeye salmon held in seawater and freshwater is uncertain, but it could include differences in states of nutrition, reproduction, and restlessness, as well as ionic differences. Regardless, this study elucidates some of the metabolic costs involved during the migration of adult salmon from seawater to freshwater, which may have applications for fisheries conservation and management models of energy use.