Perhaps the greatest source of intraspecific cardiac plasticity has yet to be discovered because comparisons among fish stocks are limited. Nonetheless, the few studies that exist often show important differences in cardiac design and function. As described above, certain stocks of domesticated rainbow trout have hearts with a greater hypoxia tolerance than others. Graham and Farrell (1992
) showed that lake-dwelling rainbow trout have large ventricles and moderate amounts of compact myocardium, whereas anadromous fish had smaller ventricles (by 10–25%) and higher levels (by 30–40%) of compact myocardium. Likewise, the Pacific salmon have appreciable stock-specific differences in both swimming performance and the optimum temperature for aerobic activity (Tsuyuki and Williscroft, 1977; Taylor and McPhail, 1985; Taylor and Foote, 1991; Lee et al., 2003b), that appear to reflect the large differences in river migratory distances, which can be as little as 100 km and as large as 1000 km. In Iceland, numerous Arctic charr morphs exist that differ substantially in morphology, behaviour and life-history characteristics, and the relative ventricular mass of an anadromous population was 10–20% greater than that of a landlocked population (M. A M. Ruiz and H. Thorarensen, unpublished data). This relationship between life history and heart size is opposite to that reported for Pacific salmon (Graham and Farrell, 1992), suggesting that natural selection has provided two contrasting model systems to examine intraspecific plasticity at the stock level.
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