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First published online May 24, 2005
Journal of Experimental Biology 208, 2055-2062 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01602

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Influence of local adaptation and interstock hybridization on the cardiovascular performance of largemouth bass Micropterus salmoides

Steven J. Cooke^* and David P. Philipp

Department of Natural Resources and Environmental Sciences, University of Illinois, and Center for Aquatic Ecology, Illinois Natural History Survey, 607 East Peabody Drive, Champaign, IL 61820, USA

^* Author for correspondence at current address: Centre for Applied Conservation Research, Department of Forest Sciences, University of British Columbia, Forest Sciences Center, 2424 Main Mall, Vancouver, British Columbia, Canada, V6T 1Z4 (e-mail: scooke{at}interchange.ubc.ca)

Accepted 15 March 2005

In this study, the cardiovascular response to exhaustive exercise among differentiated stocks of largemouth bass Micropterus salmoides was compared at 10°C and 20°C to assess the level of their local adaptation. In addition, the impact that interstock hybridization had on adaptive differences was assessed using F₁ hybrids. To accomplish these assessments, four genetically distinct stocks of fish were produced using adults from two regions in the midwestern United States identified as distinct conservation management units (central Illinois, IL and southeastern Wisconsin, WI): both P₁ stocks and both reciprocal F₁ interstock hybrids. Cardiac variables (both resting and maximal) were consistently lowest for the ILxIL stock relative to the WIxWI stock and both F₁ interstock hybrids. Interestingly, however, all groups of fish were able to maintain similar levels of cardiac scope. All fish responded to exercise by increasing heart rate and decreasing stroke volume, consistent with the notion that largemouth bass modulate cardiac output via frequency. After exercise, cardiac variables returned to resting levels 25-35% more rapidly for ILxIL fish relative to all other groups at 20°C. At 10°C, recovery rates for both P₁ stocks were similar but more rapid than the interstock hybrids. Collectively, these results indicate that the locally adapted stock (ILxIL) exhibited cardiovascular adaptations that enabled rapid cardiovascular recovery and maintenance of low resting cardiac output and heart rate. Conversely, the translocated stock (WIxWI) and the interstock hybrids required longer for cardiovascular variables to recover after exercise and exhibited higher resting levels of cardiac output and heart rate. This study provides some of the first direct evidence of a physiological mechanism by which mixing stocks could potentially decrease fitness and illustrates the magnitude of the intraspecific diversity of cardiovascular performance.

Key words: cardiac output, heart rate, local adaptation, outbreeding, translocation, largemouth bass, Micropterus salmoides