The Adaptive Response of Salmon to Global Warming

…the extinction horrors of climate change may be a “fish story”

Perhaps the myth-iest chestnut in the scary global warming meme is that our dear earth’s panoply of species is adapted only to the current climatic regime, and changing that regime means certain death, i.e. extinction.

That’s an easy, simplistic sell, but it denies some of the subtleties of organismal biology. Four decades ago, scientists realized that evolution has preserved a variety of responses to environmental change. It turns out that our enzymes, the basic material that catalyze life as we know it, actually change their shape as climate changes. Whether this is because we have so much information stored in our DNA that has survived countless generations and a variety of climates, or whether the response is simply built into the enzymes is unknown, but it is ubiquitous. It even has a catchy name: “Phenotypic Plasticity.”

Before your eyes glaze over, a little explanation is in order.

Each one of us has a genotype, which is our DNA, and each of us has an expression of that, our “phenotype.” Obviously not all genes express themselves—if they did, our physiological destiny would be eminently predictable, but it is not. Instead, we all carry strands of DNA that could theoretically cause major disease that generally do not express (or “penetrate” in the lingo of biology), and we also have DNA that could probably defeat many of the aging processes, that similarly do not express.

Instead, organisms display “plastic” responses when their environment changes. And so, species-related concerns over potential CO2-induced global warming may be dramatically overblown. And, though they don’t get much publicity, scientists are continually documenting our amazing adaptability.

Consider one of our most important marine sources of food: the salmon family. What happens when the oceans warm? In the words of Anttila et al. (2014), “a population has the options of either [1] migrating to more suitable environments (if any are available and accessible), [2] acclimating to the new temperature by exploiting its phenotypic plasticity, or [3] adapting through natural selection.” Recognizing these options, Anttila et al. set out to investigate which of these paths Atlantic salmon (Salmo salar) might pursue in response to future increases in temperature.

To achieve their objective, the team of seven researchers gathered specimens of two wild Atlantic salmon populations from the northern (coolest) and southern (warmest) extremes of their European distribution, which range spans a distance of over 3,000 km. Eggs from both groups were hatched in a salmon nursery and thereafter the juveniles were acclimated for three months at temperatures of either 12°C or 20°C. The salmon were then evaluated and tested for cardiac performance, as “cardiac function has been observed to limit the tolerance to high temperatures.” This was accomplished by subjecting the salmon to temperatures well above their acclimated state, whereupon their cardiac performance was evaluated.

In describing their findings, the seven scientists report the salmon populations “differed very little in their acute cardiac response to temperature, but instead showed considerable cardiac plasticity in response to thermal acclimation that surprisingly was largely independent of the latitudinal and climatic origin of the populations.” In other words, regardless of the acclimation temperature, 12°C or 20°C, both salmon populations exhibited a similar stress response as temperatures increased. They also found that acclimation to 20°C consistently raised the temperature at which various measures of acute cardiac stress were observed. For example, they write “although cardiac collapse starts at 21°C-23°C with a maximum heart rate of ~150 beats per minute (bpm) for 12°C-acclimated fish, acclimation to 20°C considerably raises this temperature (27.5°C) and maximum heart rate (~200 bpm).”

The results of the Anttila et al.’s analysis indicate that the response of Atlantic salmon to temperature stress–-as evaluated by cardiac performance-–is “largely dependent on individual thermal history and largely independent of local adaptation,” as offspring of both populations displayed phenotypic plasticity in adapting to the two acclimation temperatures. Such findings are encouraging, as the researchers state they “emphasize that acclimation remains a feasible possibility for survival in a warmer future, with physiological plasticity replacing the immediate need for local adaptation,” adding that “this plasticity might aid northern Atlantic salmon populations to compensate for a warmer future.”

Although this response represents only one of the three options by which to face the challenge of potential future global warming, it appears to be more than sufficient to overcome the worst possible scenarios. In addition, Anttila et al. optimistically add “natural selection has the potential to improve thermal tolerance in Atlantic salmon beyond the demonstrated benefits of high thermal plasticity,” particularly through transgenerational changes in temperature tolerance in which the heritability of thermal tolerance is passed down from parents to offspring.

All in all, therefore, the future looks bright for Atlantic salmon!

Reference: Anttila, K., Couturier, C.S., Overli, O., Johnsen, A., Marthinsen, G., Nilsson, G.E. and Farrell, A.P. 2014. Atlantic salmon show capability for cardiac acclimation to warm temperatures. Nature Communications 5: 10.1038/ncomms5252.