When do fish succumb to heat?

doi:10.1126/science.abd1272

GSTDTAP > 气候变化

DOI	10.1126/science.abd1272
	When do fish succumb to heat?
	Jennifer Sunday
	2020-07-03
发表期刊	Science
出版年	2020
英文摘要	Physiological responses to temperature can provide a window into climate change vulnerability of species. How warm will it get relative to a species' ability to tolerate heat? One complication in answering this question is that an organism's temperature tolerance can change throughout its life span, and organisms in early life stages may be more sensitive to cold and heat extremes than adults (the stage commonly measured) ([ 1 ][1], [ 2 ][2]). On page 65 of this issue, Dahlke et al. ([ 3 ][3]) report comparisons of the thermal tolerance limits of almost 700 freshwater and marine fish species at four life stages. Their findings confirm that embryos, and the reproductive adults who produce them, tolerate narrower temperature extremes than larvae and nonspawning adults, with thermal breadths (the difference between upper and lower thermal tolerance) that are narrower by an average of 20°C. As a result of this difference, projected climate vulnerabilities of fishes are greater than previously thought. Dahlke et al. constrained each fish species' geographic range to the locations in which spawners and embryos could survive, and then asked how much hotter it can get before that life stage is squeezed out. They did this by calculating the difference between the upper thermal limit of the most sensitive life stage and the mean maximum environmental temperature in that part of its range. The authors found that by the year 2100, in an emissions scenario consistent with current political commitments, 40% of the fish species examined could not exist in the current geographic range of their most sensitive life stage. This projection can be reduced to ~10% if ambitious action is taken to reduce emissions. Still, these values are notably greater than the number of species projected to be out of range when the authors used only adult thermal tolerance (fewer than 5% of species). Indeed, most previous studies have analyzed thermal limits of only larvae and adults ([ 4 ][4]–[ 6 ][5]). Hence, sensitivities may have been underestimated. Why are embryos and reproductive adults less tolerant to extreme temperatures? These findings align with predictions under the oxygen-limited thermal tolerance hypothesis, which postulates that the tolerance of aquatic ectotherms to temperature extremes is driven by an outpacing of oxygen demand relative to supply at extreme cold and hot temperatures ([ 1 ][1], [ 2 ][2]). It simply gets too cold or hot to breathe. During development from egg to adult, fish increase their aerobic capacity through the development of a cardiorespiratory system that improves the supply of oxygen to tissues, and adult thermal tolerance increases. During reproduction, adult fish increase their biomass and oxygen demand, without a simultaneous increase in oxygen supply. As a result, say Dahlke et al. , the adult's thermal tolerance decreases. Their finding that embryos and spawning adults have both reduced heat and cold tolerance lends strong support to the oxygen-limited thermal tolerance hypothesis. What does this mean for fish? The greater sensitivity of eggs and reproductive adults means that marine and freshwater fish are living much closer to their thermal limits than previously thought. Climate change is expected to provide strong selective pressure for spawning ranges of fish to move if they can, and if not, declines will be greater than predicted. However, it must be pointed out that fish populations are dynamic and have compensatory mechanisms that may disguise the survival response of any one life stage. For example, the predominance of negative density-dependent processes that regulate larval and adult survival (e.g., competition) means that populations of adults may not be altered even if many embryos fail, as long as some survive ([ 7 ][6]). In other words, per capita survival of larvae and adults can increase when there are fewer of them, compensating the adult population. Such compensatory dynamics mean that signs of dangerous spawning habitat loss in adult fish populations may not be detected until there are severe declines. The fish species assessed by Dahlke et al. cover a broad range of fish life histories, across which the immediate impacts of declining spawning habitats can be expected to vary. Stage-structured life history models will be a key asset to contextualize these important new findings. Dahlke et al. also tested the hypothesis that metabolic responsiveness to temperature is greater in fish species and life stages that have narrower thermal tolerance ranges. Ectotherms have a characteristic exponential increase in their metabolic rate with temperature ([ 8 ][7]). The parameter that defines the shape of this relationship—the activation energy—is thought to reflect universal kinetic barriers that limit the rate of reactions, which in turn constrain higher-level rates such as oxygen consumption, growth rate, and development rate ([ 8 ][7]). By estimating this parameter, Dahlke et al. found that organisms with narrower thermal tolerance breadth (stenotherms) have higher activation energies than those with broad thermal tolerance (eurytherms). These results suggest that activation energies do not strictly conform to statistical thermodynamics [with a central tendency of ∼0.60 to 0.70 eV ([ 9 ][8])] but can be modified through selection and trade-offs. Understanding variation in activation energy as a function of thermal breadth may provide opportunities to predict higher-level ecological responses to climate warming. For example, relative activation energies of individual species can be used to predict temperature-related changes in trophic interactions and ecosystem stability ([ 10 ][9]). Predictions of relative biomass changes with temperature may now plausibly be applied to systems where, for example, eurytherms interact with stenotherms, or where the adults of one species feed on the eggs of another. Variation in activation energies can also be directly entered into dynamic size-based food web models that predict temperature effects on fish productivity ([ 11 ][10]). One key opportunity for future work is to improve coverage of comparable thermal tolerance data across life stages. Dahlke et al. built their dataset by compiling all of the thermal tolerance data that are publicly available, and then used phylogenetic imputation to fill in the unknown values according to a phylogeny and best-fitting model of evolution. Although this has a precedent in the analyses of fish thermal limits ([ 5 ][11]), it is important to realize that when interpreting the results, many of the values shown by Dahlke et al. were not observed in their study. Similarly, some metrics of thermal tolerance differed systematically among life stages, because the more comparable thermal limit metrics were not available. Given the ramifications of these results, it will be critical to build a dataset in which thermal tolerance limits of embryos and spawning adults are estimated using the same methods as for larvae and nonspawning adults. Fish are an increasingly important source of protein for human consumption as the global population grows ([ 12 ][12]), and the finding of greater sensitivity in eggs and reproductive adults on the order of 20°C is major cause for concern. The minute thermal safety margins of spawning fish and embryos in the tropics suggest that there are limited fish species on Earth that can tolerate warmer or less oxygenated habitats. Intensified efforts to stabilize global warming are warranted more than ever. 1. [↵][13]1. H.-O. Pörtner, 2. R. Knust , Science 315, 95 (2007). [OpenUrl][14][Abstract/FREE Full Text][15] 2. [↵][16]1. H.-O. Pörtner , J. Exp. Biol. 213, 881 (2010). [OpenUrl][17][Abstract/FREE Full Text][18] 3. [↵][19]1. F. T. Dahlke, 2. S. Wohlrab, 3. M. Butzin, 4. H.-O. Pörtner , Science 369, 65 (2020). [OpenUrl][20][Abstract/FREE Full Text][21] 4. [↵][22]1. M. L. Pinsky, 2. A. M. Eikeset, 3. D. J. McCauley, 4. J. L. Payne, 5. J. M. Sunday , Nature 569, 108 (2019). [OpenUrl][23][CrossRef][24][PubMed][25] 5. [↵][26]1. L. Comte, 2. J. D. Olden , Proc. R. Soc. B 285, 20172214 (2018). [OpenUrl][27][CrossRef][28][PubMed][29] 6. [↵][30]1. C. H. Trisos, 2. C. Merow, 3. A. L. Pigot , Nature 580, 496 (2020). [OpenUrl][31][PubMed][32] 7. [↵][33]1. H. K. Kindsvater, 2. M. Mangel, 3. J. D. Reynolds, 4. N. K. Dulvy , Ecol. Evol. 6, 2125 (2016). [OpenUrl][34] 8. [↵][35]1. J. H. Brown, 2. J. F. Gillooly, 3. A. P. Allen, 4. V. M. Savage, 5. G. B. West , Ecology 85, 1771 (2004). [OpenUrl][36][CrossRef][37][Web of Science][38] 9. [↵][39]1. J. F. Gillooly, 2. J. H. Brown, 3. G. B. West, 4. V. M. Savage, 5. E. L. Charnov , Science 293, 2248 (2001). [OpenUrl][40][Abstract/FREE Full Text][41] 10. [↵][42]1. B. Gilbert et al ., Ecol. Lett. 17, 902 (2014). [OpenUrl][43][CrossRef][44][PubMed][45] 11. [↵][46]1. J. L. Blanchard et al ., Philos. Trans. R. Soc. B 367, 2979 (2012). [OpenUrl][47][CrossRef][48][PubMed][49] 12. [↵][50]Food and Agriculture Organization of the United Nations, The State of World Fisheries and Aquaculture 2016 (2016). Correction (2 July 2020): The second half of the second paragraph was changed before publication, but the printed magazine still includes the previous version of this text. The online HTML and PDF are correct. 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/281866
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Jennifer Sunday. When do fish succumb to heat?[J]. Science,2020.
APA	Jennifer Sunday.(2020).When do fish succumb to heat?.Science.
MLA	Jennifer Sunday."When do fish succumb to heat?".Science (2020).