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

In this study, the cardiovascular response to exhaustive exercise among differentiated stocks of largemouth bass Micropterus salmoides was compared at 10 degrees C and 20 degrees 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 IL x IL stock relative to the WI x WI 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 IL x IL fish relative to all other groups at 20 degrees C. At 10 degrees 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 (IL x IL) exhibited cardiovascular adaptations that enabled rapid cardiovascular recovery and maintenance of low resting cardiac output and heart rate. Conversely, the translocated stock (WI x WI) 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.