| 英文摘要: | To the Editor —
Muñoz et al.1 present some interesting and valuable experimental data about the physiological responses of chinook salmon (Oncorhynchus tshawytscha) to changes in developmental temperature. Especially notable is the way they develop quantitative genetic data to evaluate the adaptive potential of cardiac performance to different temperature regimes. Pacific salmon clearly have the ability to develop population-specific adaptations in cardiac performance over evolutionary time scales2, but they found relatively little capacity for adaptive genetic or plastic responses in one key performance measure, the arrhythmic temperature, in the population they studied. However, we raise concerns about their extrapolation from a small study to broad conclusions about vulnerability of the entire species to climate change. They claim that “rising temperatures now threaten the persistence of [salmon]”1. While it is true that many individual salmon populations and some regional population groups are at risk, threats to persistence are multifaceted and population-specific3. Moreover, the premise that persistence of the genus, or any one of the Oncorhynchus species, is now threatened by rising temperature is not supported by other empirical evidence. We are also concerned that this study ignored the documented capability of salmonids to respond to environmental change with plastic and evolutionary changes in behaviour, such as upstream (adult) and downstream (juvenile) migration timing4. Changes in phenology, rather than physiological tolerances, provide greater capacity for resilience to climate change in salmonids5 and other taxa more generally6, although the two clearly interact and the relative importance of behavioural and physiological responses may vary across taxa or contexts (such as geographic locations)7.
The Quinsam River population used in the experiment by Muñoz et al. inhabits a watershed where average water temperatures seasonally increase by about 7 °C from April to June8. Warmer temperatures are associated with more rapid development and earlier downstream migration for juvenile salmonids4. If juvenile migration advanced by a month, which is well-within the capacity for a rapid plastic response in chinook salmon, this behavioural shift would prevent exposure to 4 °C of warming in June temperature. A four-year study of juvenile migration timing in chinook salmon in Oregon's Umpqua River showed that median migration dates in two tributaries advanced 40 days when spring water temperatures were 5 °C higher9. Given that egg development is also likely to be advanced under warmer temperatures (Muñoz et al. themselves found that entry into the juvenile phase occurred 50 days earlier in their +4 °C treatment group), juvenile development in fresh water need not be cut short by earlier ocean entry. Of course, timing changes at any one life stage may pose fitness challenges in subsequent life stages, so advanced ocean migration timing might come with fitness costs10.
In addition to immediate plastic responses, evolution of reaction norms (the phenotypic response of a single genotype across a range of environments), as observed in Columbia River sockeye salmon (O. nerka) adult migration timing11, allows further modification of these traits. Taking evolution in adult migration timing into account greatly reduces the simulated probability of extinction in a population of sockeye salmon in the Fraser River, British Columbia, Canada, under climate warming scenarios10 (Fig. 1). |