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If Earth's Oceans Warm, Will Cold-Water Salmon Disappear?

Paper Reviewed
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.

Among the many concerns over potential CO2-induced global warming is the prospect that rising temperatures will drive large populations of certain animal species toward extinction. Animals living at the warm end of their distribution, for example, are frequently assumed to be living near their thermal maximum and, as such, are unlikely to adapt if temperatures increase above that range.

In facing such a challenge, however, Anttila et al. (2014) write that "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. recently conducted an experiment 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 young juveniles were acclimated for 3 months at temperatures of either 12 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 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 that "although cardiac collapse starts at 21-23°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 worse possible scenarios. In addition, Anttila et al. optimistically add that "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!

Posted 6 November 2014