Muñoz N.J., Farrell A.P., Heath J.W., and Neff B.D. Adaptive potential of a Pacific Salmon challenged by climate change. Nature Climate Change. 22 December, 2014. doi: 10.1038/NCLIMATE2473
When you and I get too cold we pile on layers. When it gets too hot we strip down, have a cool drink, or sit in an air-conditioned room. Lucky you weren’t born a fish. When faced with increasing water temperatures, fish rely on the ability of their hearts to work harder to distribute less oxygen more quickly. So, what’s the breaking point? How far can a fish push its “cardiorespiratory” faculties before it can simply no longer survive? A study by Muñoz et al. published in Nature Climate Change warns of potential catastrophic effects on the population of Pacific salmon that just can’t cope with global warming.
Two main mechanisms influence a population’s resilience to environmental changes such as increasing water temperatures, namely, the innate ability of the individual to acclimatize (or “phenotypic plasticity), and, in the longer term, the ability of populations to adapt genetically through natural selection. For many marine animals, the ability to acclimatize or adapt to increased temperature comes down to cardiac performance, which peaks at an optimal temperature and declines thereafter due to limitations of the heart’s ability to pump blood quickly enough. This might not be such a big problem for fish with relatively low aerobic demands, such as those that ambush their prey from the shadows, but salmon are not lazy fish, making them an especially vulnerable to thermal stress.
To test for phenotypic plasticity, indicators for cardiac performance were compared for individuals raised at average present-day river temperatures to those reared at an elevated temperature (+4 ºC) intended to mimic warming projections. By most indications, the +4 ºC population exhibited significant plasticity in cardiac performance. That is, individuals were able to up their game enough to increase their temperature optima, as well as heart rates and temperatures of maximal heart rate. However, when it came down to the temperature of onset of arrhythmias, irregular heartbeats indicative of imminent cardiac failure, a temperature ceiling was observed independent of temperature treatment, implying that on a given day in the future, if temperatures were to exceed this limit, all bets would be off for Pacific salmon.
Additionally, Muñoz et al. examined evolutionary potential for genetic effects to influence cardiac performance. In the equation of mating, males and females contribute half of their genetic material to produce offspring. However, the developmental environment is entirely determined by the female. Hence, Muñoz et al. reasoned that the father’s contribution should represent an “additive” genetic effect, so comparing the adaptability of paternal half-siblings could provide a means for assessing the potential of genes to influence a population’s thermal resilience. As it happened, paternity had a significant influence on temperature of optimal cardiac performance, maximum heart rate, and range of performance, suggesting that the salmon may have the potential to adapt to climate change. However, the temperature of onset of arrhythmias had more to do with maternity, which Muñoz et al. suggest relates to the ability of the mother to influence egg size, and thereby developmental environment (yours truly might further offer that mothers contribute mitochondria, which are key to metabolic performance). Paternity, on the other hand, showed no relationship with environmental treatment, with the exception of an almost believable correlation to the temperature of optimal cardiac performance, suggesting that some genotypes might be more phenotypically talented than others.
Muñoz et al. conclude their study with macabre predictions of the potential for catastrophic collapse of salmon populations. Using climate projections put forth the by the International Panel on Climate Change, they modeled the chance of temperatures to exceed arrhythmia temperatures for a given day from the present to 2100. Average warming predictions foretell a 5% chance of loss by 2075, climbing to 55% by 2100, a coin toss that’s slightly loaded against fish. Under the most pessimistic of warming predictions these numbers shoot up to 17% by 2075 or 98% by 2100, near certain demise.
The study by Muñoz et al. provides further evidence of the risks posed by climate change, as well as emphasizing the importance of physiological studies that focus on individual species’ abilities to cope with thermal stress. Not all species are the same, even if they are highly related. For example, Atlantic salmon (endangered for other anthropogenic reasons) exhibit plasticity with respect to the onset temperature of arrhythmia. If researchers simply decided that all salmon were the same, the true risk of climate change on more vulnerable Pacific salmon would go unnoticed until catastrophic loss would become a reality. Whether its extinction of a bagel and lox, or the potential for ecological catastrophe that gets you going, population collapse of Pacific salmon could be seriously bad news. If the fate of Pacific salmon is worth more than a coin toss, something must be done to mitigate the effects of climate change.