| 英文摘要: | Pacific salmon provide critical sustenance for millions of people worldwide and have far-reaching impacts on the productivity of ecosystems. Rising temperatures now threaten the persistence of these important fishes1, 2, yet it remains unknown whether populations can adapt. Here, we provide the first evidence that a Pacific salmon has both physiological and genetic capacities to increase its thermal tolerance in response to rising temperatures. In juvenile chinook salmon (Oncorhynchus tshawytscha), a 4 °C increase in developmental temperature was associated with a 2 °C increase in key measures of the thermal performance of cardiac function3, 4. Moreover, additive genetic effects significantly influenced several measures of cardiac capacity, indicative of heritable variation on which selection can act. However, a lack of both plasticity and genetic variation was found for the arrhythmic temperature of the heart, constraining this upper thermal limit to a maximum of 24.5 ± 2.2 °C. Linking this constraint on thermal tolerance with present-day river temperatures and projected warming scenarios5, we predict a 17% chance of catastrophic loss in the population by 2100 based on the average warming projection, with this chance increasing to 98% in the maximum warming scenario. Climate change mitigation is thus necessary to ensure the future viability of Pacific salmon populations. Rapid increases in temperature due to anthropogenic climate change are projected to have extensive impacts on natural systems due to the profound effects of temperature on organisms6. Indeed, species are adapted to their thermal environment at scales both local and global, such that breadths of thermal tolerance and thermal optima for performance often correspond to the thermal conditions in which species evolved7. The capacities for adaptation and acclimation are key determinants of how populations can cope with climate change, as they allow phenotypes to ‘track’ a changing environment8, 9, 10. In fishes, most phenotypic responses to climate change that have been documented are attributable to phenotypic plasticity9, although genetic responses within populations are also expected to occur due to changes in selection pressures10. The degree to which both these processes can act on functional traits that set thermal tolerance will greatly influence the ability of populations to inhabit more extreme environments. In many aquatic ectotherms such as fish, the limits of thermal tolerance are thought to be set by a mismatch between oxygen supply and demand11. Aerobic scope—the difference between an organism’s minimum and maximum oxygen consumption rate—peaks at an optimum temperature (Topt) and subsequently declines with further warming owing to capacity limitations of the cardiorespiratory system12. The limits of thermal tolerance are reached when insufficient scope is available for key aerobic activities such as swimming, growth or reproduction. Although key for active species such as the Pacific salmon (Oncorhynchus spp.)1, this loss of aerobic scope may be less important for some species, such as benthic ambush predators that have minimal aerobic demands13, 14. Yet, increased mortality in both Pacific salmon1 and benthic eelpout (Zoarces viviparus)11 populations has been linked with a loss of aerobic scope during anomalously high temperatures. Such loss of aerobic scope is largely driven by limitations on maximum heart rate (fHmax), as increased heart rate is the primary mechanism that supports increased tissue oxygen demand at higher temperature15. Indeed, differences in the thermal performance of cardiac function explain patterns of biogeography in marine intertidal invertebrates16, reef fish17 and pelagic predatory fish18, whereas the correspondence between Topt and local thermal conditions among Pacific salmon populations corresponds with differences in cardiac capacity19. Although these differences suggest a potential for thermal tolerance to track environmental temperatures, the potential for both adaptation and acclimation of cardiac capacity remains largely unknown. Studies of the heritability and plasticity of oxygen-limited thermal tolerance are thus needed to understand how populations might cope with rapid climate change. High river temperatures have recently been linked with increased mortality of juvenile chinook salmon (O. tshawytscha)20, raising concerns over the future viability of this ecologically and economically important species2. To assess the extent to which chinook salmon can adapt or acclimate to rising temperatures, we mated wild-caught adults in full-factorial crosses21 and reared offspring from each family in present-day (+0 °C) and projected future (+4 °C) temperature conditions. We then measured the response of fHmax to warming3 in juvenile offspring from each family and temperature treatment. We found that the Arrhenius break temperature of fHmax (TAB), which corresponds to Topt (refs 3, 4), averaged 14.0 ± 1.1°C in the +0 °C group and 16.1 ± 0.9 °C in the +4 °C group (Fig. 1 and Supplementary Fig. 1). The peak fHmax (fHpeak) averaged 153 ± 18 beats min−1 in the +0 °C group and 180 ± 17 beats min−1 in the +4 °C group. The average temperature at which fHpeak was reached (TpeakfH) was 20.8 ± 2.3 °C and 22.8 ± 1.9 °C in the +0 °C and +4 °C groups, respectively, whereas the arrhythmic temperature of fHmax (Tarr), which signifies the onset of cardiac failure and which corresponds well with the upper thermal limit for aerobic performance3, 4, was similar in the two groups at 24.2 ± 1.6 °C and 24.5 ± 2.2 °C, respectively. Thus, the average TAB and TpeakfH increased ~2 °C after developmental acclimation to future temperatures, whereas Tarr increased only 0.3 °C. Indeed, Tarr was the only trait that did not significantly differ between treatment groups (Table 1), similar to previous findings that warm acclimation provides little benefit to upper temperature tolerance in Pacific salmon22. Still, our results indicate that chinook salmon can plastically increase the maximum capacity of their hearts (fHpeak) as well as TAB and TpeakfH. This result differs from wild Atlantic salmon, which can adjust Tarr in addition to fHpeak, TAB and TpeakfH in response to warm acclimation23.
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![Mean increase in maximum heart rate (fHmax) among all offspring from the +0 [deg]C (black line) and +4 [deg]C (grey line) treatment groups of Quinsam River chinook salmon (O. tshawytscha).](http://www.nature.com/nclimate/journal/v5/n2/images_article/nclimate2473-f1.jpg)